diff --git a/doc/mkdwarfs.md b/doc/mkdwarfs.md
index 4441b6be..b8a1791c 100644
--- a/doc/mkdwarfs.md
+++ b/doc/mkdwarfs.md
@@ -242,13 +242,19 @@ Most other options are concerned with compression tuning:
 - `--rebuild-metadata`:
   Completely rebuild the metadata block. This will upgrade the internal format
   of the metadata to the latest version instead of just recompressing the
-  metadata block.
+  metadata block. Implies `--recompress=metadata`.
+
+- `--change-block-size`:
+  Change the block size while recompressing. This will change the block size
+  according to the size given in `--block-size-bits`. Even if the block size
+  is unchanged, this will still re-order and re-compress *all* blocks. Implies
+  `--recompress=all` and `--rebuild-metadata`.
 
 - `--recompress-categories=`[`!`]*category*[`,`...]:
   When `--recompress` is set to `all` or `block`, this option controls
   which categories of blocks will be recompressed. Adding a `!` in front
   of the list allows you to specify which categories will *not* be
-  recompressed.
+  recompressed. Cannot be used with `--change-block-size`.
 
 - `-P`, `--pack-metadata=auto`|`none`|[`all`|`chunk_table`|`directories`|`shared_files`|`names`|`names_index`|`symlinks`|`symlinks_index`|`force`|`plain`[`,`...]]:
   Which metadata information to store in packed format. This is primarily
diff --git a/include/dwarfs/reader/filesystem_v2.h b/include/dwarfs/reader/filesystem_v2.h
index deeac448..42b09ef2 100644
--- a/include/dwarfs/reader/filesystem_v2.h
+++ b/include/dwarfs/reader/filesystem_v2.h
@@ -487,6 +487,9 @@ class filesystem_v2 final : public filesystem_v2_lite {
 
   std::unique_ptr<thrift::metadata::fs_options> thawed_fs_options() const;
 
+  std::future<block_range>
+  read_raw_block_data(size_t block_no, size_t offset, size_t size) const;
+
   class impl : public impl_lite {
    public:
     virtual int
@@ -504,6 +507,8 @@ class filesystem_v2 final : public filesystem_v2_lite {
     unpacked_metadata() const = 0;
     virtual std::unique_ptr<thrift::metadata::fs_options>
     thawed_fs_options() const = 0;
+    virtual std::future<block_range>
+    read_raw_block_data(size_t block, size_t offset, size_t size) const = 0;
   };
 
  private:
diff --git a/include/dwarfs/reader/internal/inode_reader_v2.h b/include/dwarfs/reader/internal/inode_reader_v2.h
index 75e47bfa..0b569536 100644
--- a/include/dwarfs/reader/internal/inode_reader_v2.h
+++ b/include/dwarfs/reader/internal/inode_reader_v2.h
@@ -105,6 +105,11 @@ class inode_reader_v2 {
     impl_->cache_blocks(blocks);
   }
 
+  std::future<block_range>
+  read_raw_block_data(size_t block_no, size_t offset, size_t size) const {
+    return impl_->read_raw_block_data(block_no, offset, size);
+  }
+
   class impl {
    public:
     virtual ~impl() = default;
@@ -127,6 +132,8 @@ class inode_reader_v2 {
     virtual void set_cache_tidy_config(cache_tidy_config const& cfg) = 0;
     virtual size_t num_blocks() const = 0;
     virtual void cache_blocks(std::span<size_t const> blocks) const = 0;
+    virtual std::future<block_range>
+    read_raw_block_data(size_t block_no, size_t offset, size_t size) const = 0;
   };
 
  private:
diff --git a/include/dwarfs/utility/rewrite_options.h b/include/dwarfs/utility/rewrite_options.h
index 26794644..d3e21e25 100644
--- a/include/dwarfs/utility/rewrite_options.h
+++ b/include/dwarfs/utility/rewrite_options.h
@@ -36,6 +36,7 @@ namespace dwarfs::utility {
 struct rewrite_options {
   bool recompress_block{false};
   bool recompress_metadata{false};
+  std::optional<size_t> change_block_size;
   std::optional<writer::metadata_options> rebuild_metadata;
   std::unordered_set<std::string> recompress_categories;
   bool recompress_categories_exclude{false};
diff --git a/include/dwarfs/writer/internal/filesystem_writer_detail.h b/include/dwarfs/writer/internal/filesystem_writer_detail.h
index 6030fa6a..0d783be2 100644
--- a/include/dwarfs/writer/internal/filesystem_writer_detail.h
+++ b/include/dwarfs/writer/internal/filesystem_writer_detail.h
@@ -32,6 +32,8 @@
 #include <utility>
 #include <vector>
 
+#include <folly/Function.h>
+
 #include <dwarfs/block_compressor.h>
 #include <dwarfs/byte_buffer.h>
 #include <dwarfs/compression_constraints.h>
@@ -48,6 +50,15 @@ class fs_section;
 
 namespace writer::internal {
 
+struct block_compression_info {
+  size_t uncompressed_size{};
+  std::optional<std::string> metadata;
+  std::optional<compression_constraints> constraints;
+};
+
+using delayed_data_fn_type = folly::Function<
+    std::pair<shared_byte_buffer, std::optional<std::string>>()>;
+
 class filesystem_writer_detail {
  public:
   virtual ~filesystem_writer_detail() = default;
@@ -81,11 +92,15 @@ class filesystem_writer_detail {
   virtual void write_history(shared_byte_buffer data) = 0;
   virtual void check_block_compression(
       compression_type compression, std::span<uint8_t const> data,
-      std::optional<fragment_category::value_type> cat = std::nullopt) = 0;
+      std::optional<fragment_category::value_type> cat = std::nullopt,
+      block_compression_info* info = nullptr) = 0;
   virtual void write_section(
       section_type type, compression_type compression,
       std::span<uint8_t const> data,
       std::optional<fragment_category::value_type> cat = std::nullopt) = 0;
+  virtual void rewrite_block(
+      delayed_data_fn_type data, size_t uncompressed_size,
+      std::optional<fragment_category::value_type> cat = std::nullopt) = 0;
   virtual void write_compressed_section(dwarfs::internal::fs_section const& sec,
                                         std::span<uint8_t const> data) = 0;
   virtual void flush() = 0;
diff --git a/include/dwarfs/writer/internal/metadata_builder.h b/include/dwarfs/writer/internal/metadata_builder.h
index 800508f7..9ee561ef 100644
--- a/include/dwarfs/writer/internal/metadata_builder.h
+++ b/include/dwarfs/writer/internal/metadata_builder.h
@@ -49,6 +49,19 @@ class inode_manager;
 class block_manager;
 class dir;
 
+struct block_chunk {
+  size_t block{};
+  size_t offset{};
+  size_t size{};
+};
+
+struct block_mapping {
+  size_t old_block{};
+  std::vector<block_chunk> chunks{};
+
+  std::vector<block_chunk> map_chunk(size_t offset, size_t size) const;
+};
+
 class metadata_builder {
  public:
   // Start with empty metadata
@@ -125,6 +138,10 @@ class metadata_builder {
     impl_->gather_global_entry_data(ge_data);
   }
 
+  void remap_blocks(std::span<block_mapping const> mapping) {
+    impl_->remap_blocks(mapping);
+  }
+
   thrift::metadata::metadata const& build() { return impl_->build(); }
 
   class impl {
@@ -152,6 +169,7 @@ class metadata_builder {
     gather_entries(std::span<dir*> dirs, global_entry_data const& ge_data,
                    uint32_t num_inodes) = 0;
     virtual void gather_global_entry_data(global_entry_data const& ge_data) = 0;
+    virtual void remap_blocks(std::span<block_mapping const> mapping) = 0;
 
     virtual thrift::metadata::metadata const& build() = 0;
   };
diff --git a/src/reader/filesystem_v2.cpp b/src/reader/filesystem_v2.cpp
index 978a1349..6eaaa89e 100644
--- a/src/reader/filesystem_v2.cpp
+++ b/src/reader/filesystem_v2.cpp
@@ -345,6 +345,11 @@ class filesystem_ final {
     return metadata_v2_utils(meta_).thaw_fs_options();
   }
 
+  std::future<block_range>
+  read_raw_block_data(size_t block_no, size_t offset, size_t size) const {
+    return ir_.read_raw_block_data(block_no, offset, size);
+  }
+
  private:
   filesystem_parser make_fs_parser() const {
     return filesystem_parser(mm_, image_offset_, options_.image_size);
@@ -1387,6 +1392,10 @@ class filesystem_full_
   thawed_fs_options() const override {
     return fs().thawed_fs_options();
   }
+  std::future<block_range> read_raw_block_data(size_t block_no, size_t offset,
+                                               size_t size) const override {
+    return fs().read_raw_block_data(block_no, offset, size);
+  }
 
  private:
   history history_;
@@ -1519,6 +1528,12 @@ filesystem_v2::thawed_fs_options() const {
   return full_().thawed_fs_options();
 }
 
+std::future<block_range>
+filesystem_v2::read_raw_block_data(size_t block_no, size_t offset,
+                                   size_t size) const {
+  return full_().read_raw_block_data(block_no, offset, size);
+}
+
 auto filesystem_v2::full_() const -> impl const& { return this->as_<impl>(); }
 
 } // namespace dwarfs::reader
diff --git a/src/reader/internal/inode_reader_v2.cpp b/src/reader/internal/inode_reader_v2.cpp
index 9b5479a1..de5db3f2 100644
--- a/src/reader/internal/inode_reader_v2.cpp
+++ b/src/reader/internal/inode_reader_v2.cpp
@@ -158,6 +158,9 @@ class inode_reader_ final : public inode_reader_v2::impl {
     }
   }
 
+  std::future<block_range> read_raw_block_data(size_t block_no, size_t offset,
+                                               size_t size) const override;
+
  private:
   using offset_cache_type =
       basic_offset_cache<uint32_t, file_off_t, size_t,
@@ -249,6 +252,13 @@ void inode_reader_<LoggerPolicy>::do_readahead(uint32_t inode,
   }
 }
 
+template <typename LoggerPolicy>
+std::future<block_range>
+inode_reader_<LoggerPolicy>::read_raw_block_data(size_t block_no, size_t offset,
+                                                 size_t size) const {
+  return cache_.get(block_no, offset, size);
+}
+
 template <typename LoggerPolicy>
 std::vector<std::future<block_range>>
 inode_reader_<LoggerPolicy>::read_internal(uint32_t inode, size_t const size,
diff --git a/src/utility/rewrite_filesystem.cpp b/src/utility/rewrite_filesystem.cpp
index 5cae0c30..ba101a76 100644
--- a/src/utility/rewrite_filesystem.cpp
+++ b/src/utility/rewrite_filesystem.cpp
@@ -21,8 +21,11 @@
  * SPDX-License-Identifier: GPL-3.0-only
  */
 
+#include <vector>
+
 #include <dwarfs/history.h>
 #include <dwarfs/logger.h>
+#include <dwarfs/malloc_byte_buffer.h>
 #include <dwarfs/reader/filesystem_v2.h>
 #include <dwarfs/util.h>
 #include <dwarfs/utility/rewrite_options.h>
@@ -38,6 +41,217 @@
 
 namespace dwarfs::utility {
 
+namespace {
+
+/*
+
+In order to be able to change the block size, we need to first build a list
+of all blocks, along with their categories *and* category-specific metadata.
+Only blocks in the same category and with the same metadata are eligible for
+merging. While category/metadata is mostly irrelevant for splitting, splitting
+requires us to know the compression constraints (i.e. the granularity of the
+data) so we can split at the correct boundaries.
+
+Granularity also makes splitting/merging more complicated, as we potentially
+cannot simply split a block because one of the new blocks would be larger than
+the block size. In which case we must move the excess data to the next block,
+and so on. Simpilarly, when merging blocks, we can potentially fill up the
+block with data from the next block.
+
+So, ultimately, we need to define for each block in the rewritten filesystem
+image the chunks of which it is made up. This mapping will not only be used
+to build the new blocks, but also to rebuild the metadata. In the metadata,
+both the chunks *and* the chunk table must be updated, since individual chunks
+can be either merged or split as well. If we want to be super accurate, we
+would also need to update the inode size cache; but this would only be relevant
+if we go from a really large block size to a really small one. Then again, it
+shouldn't be too hard to update the cache. What we *definitely* need to update
+is the `block_categories` as well as the `block_category_metadata` tables in
+the metadata.
+
+So, what we need:
+
+- A list of all blocks, along with their categories and metadata
+
+*/
+
+struct block_info {
+  size_t block{};
+  size_t uncompressed_size{};
+  std::optional<dwarfs::internal::fs_section> section;
+  std::optional<std::string> category_name;
+  std::optional<std::string> metadata;
+  std::optional<compression_constraints> constraints;
+};
+
+/*
+
+- An algorithm for splitting/merging that outputs the new block positions
+  (numbers) and the chunks that make up each block
+
+    struct block_chunk {  // see metadata_builder.h
+      size_t block;
+      size_t offset;
+      size_t size;
+    };
+
+*/
+
+struct new_block_mapping {
+  size_t block{};
+  size_t size{};
+  std::vector<dwarfs::writer::internal::block_chunk> chunks{};
+  std::optional<std::string> category_name;
+  std::optional<std::string> metadata;
+};
+
+/*
+
+- The algorithm should be deterministic. It doesn't have to be reversible,
+  i.e. splitting then merging or merging then splitting doesn't have to
+  yield the same result (or even the original filesystem image). But when
+  splitting or merging, the result should *always* be the same given the
+  same input. That means we *could* actually consider grouping blocks by
+  category and metadata in the output.
+
+  TODO: Check if we've gone from a compression with constraints to one
+        without (i.e. granularity 3 -> 1) and want to go back to the
+        original compression *without* a block size change, that should
+        fail early.
+
+We need two new features to support this:
+
+- `filesystem_v2` must allow reading raw block data (i.e. not file-based).
+  That way, we can easily make use of the block cache while re-composing
+  the blocks.
+
+- `filesystem_writer` must allow delayed reading of the block data. We
+  can hopefully refactor the `rewritten_fsblock` to support this.
+
+How does the remapping process work in the metadata builder?
+
+The chunk_table is just a list of the first chunk of each regular file
+inode, plus a sentinel at the end. Basically, we need to traverse the
+chunk_table and the chunks it references and build new versions of the
+chunk_table and chunks using the new blocks.
+
+To build a new chunk from an old chunk, we must be able figure out which
+new blocks an old block is mapped to. This is sort of the opposite of
+`mapped_block_info`, where we have stored which chunks of old blocks
+make up a new block. So we need a second mapping:
+
+    struct block_mapping {  // see metadata_builder.h
+      size_t old_block;
+      std::vector<block_chunk> chunks;
+    };
+
+*/
+
+struct rw_block_mappings {
+  std::vector<new_block_mapping> new_to_old;
+  std::vector<dwarfs::writer::internal::block_mapping> old_to_new;
+};
+
+rw_block_mappings build_block_mappings(std::span<block_info const> blocks,
+                                       size_t const block_size) {
+  using stream_id =
+      std::pair<std::optional<std::string>, std::optional<std::string>>;
+  std::vector<std::vector<size_t>> streams;
+  std::map<stream_id, size_t> stream_map;
+
+  for (auto const& b : blocks) {
+    stream_id id{b.category_name, b.metadata};
+    auto [it, inserted] = stream_map.try_emplace(id, streams.size());
+    if (inserted) {
+      streams.emplace_back();
+    }
+    streams[it->second].push_back(b.block);
+  }
+
+  rw_block_mappings result;
+
+  for (auto const& stream : streams) {
+    size_t granularity{1};
+
+    if (auto const& cc = blocks[stream[0]].constraints; cc && cc->granularity) {
+      granularity = cc->granularity.value();
+    }
+
+    size_t const max_stream_block_size{granularity *
+                                       (block_size / granularity)};
+
+    std::vector<new_block_mapping> mapped;
+
+    for (size_t block : stream) {
+      result.old_to_new.push_back({.old_block = block});
+      auto& old_to_new = result.old_to_new.back();
+      auto const& b = blocks[block];
+      size_t offset{0};
+
+      while (offset < b.uncompressed_size) {
+        if (mapped.empty() || mapped.back().size == max_stream_block_size) {
+          mapped.push_back({.block = result.new_to_old.size() + mapped.size(),
+                            .category_name = b.category_name,
+                            .metadata = b.metadata});
+        }
+
+        auto& m = mapped.back();
+        size_t const chunk_size{std::min(b.uncompressed_size - offset,
+                                         max_stream_block_size - m.size)};
+
+        DWARFS_CHECK(chunk_size % granularity == 0,
+                     fmt::format("chunk_size ({}) % granularity ({}) != 0",
+                                 chunk_size, granularity));
+
+        old_to_new.chunks.push_back(
+            {.block = m.block, .offset = m.size, .size = chunk_size});
+
+        m.chunks.push_back(
+            {.block = block, .offset = offset, .size = chunk_size});
+
+        m.size += chunk_size;
+        offset += chunk_size;
+      }
+    }
+
+    std::ranges::move(mapped, std::back_inserter(result.new_to_old));
+  }
+
+  std::ranges::sort(result.old_to_new, [](auto const& a, auto const& b) {
+    return a.old_block < b.old_block;
+  });
+
+  return result;
+}
+
+std::string block_mappings_to_string(rw_block_mappings const& mapped) {
+  std::ostringstream oss;
+  for (auto const& m : mapped.new_to_old) {
+    oss << "new block " << m.block << " (size " << m.size;
+    if (m.category_name) {
+      oss << ", category " << *m.category_name;
+    }
+    if (m.metadata) {
+      oss << ", metadata " << *m.metadata;
+    }
+    oss << "):\n";
+    for (auto const& c : m.chunks) {
+      oss << "  chunk: old block " << c.block << ", offset " << c.offset
+          << ", size " << c.size << "\n";
+    }
+  }
+  for (auto const& m : mapped.old_to_new) {
+    oss << "old block " << m.old_block << ":\n";
+    for (auto const& c : m.chunks) {
+      oss << "  chunk: new block " << c.block << ", offset " << c.offset
+          << ", size " << c.size << "\n";
+    }
+  }
+  return oss.str();
+}
+
+} // namespace
+
 void rewrite_filesystem(logger& lgr, dwarfs::reader::filesystem_v2 const& fs,
                         dwarfs::writer::filesystem_writer& fs_writer,
                         dwarfs::writer::category_resolver const& cat_resolver,
@@ -46,6 +260,18 @@ void rewrite_filesystem(logger& lgr, dwarfs::reader::filesystem_v2 const& fs,
 
   LOG_PROXY(debug_logger_policy, lgr);
 
+  if (opts.change_block_size) {
+    DWARFS_CHECK(opts.recompress_block,
+                 "change_block_size requires recompress_block");
+    DWARFS_CHECK(opts.recompress_metadata,
+                 "change_block_size requires recompress_metadata");
+    DWARFS_CHECK(opts.rebuild_metadata,
+                 "change_block_size requires rebuild_metadata");
+  }
+
+  std::vector<block_info> blocks;
+  rw_block_mappings mapped_blocks;
+
   auto parser = fs.get_parser();
 
   auto& writer = fs_writer.get_internal();
@@ -54,20 +280,64 @@ void rewrite_filesystem(logger& lgr, dwarfs::reader::filesystem_v2 const& fs,
     size_t block_no{0};
     parser->rewind();
 
-    while (auto s = parser->next_section()) {
-      if (s->type() == section_type::BLOCK) {
-        if (auto catstr = fs.get_block_category(block_no)) {
-          if (auto cat = cat_resolver.category_value(catstr.value())) {
-            writer.check_block_compression(s->compression(),
-                                           parser->section_data(*s), cat);
+    {
+      auto tv = LOG_TIMED_VERBOSE;
+
+      while (auto s = parser->next_section()) {
+        if (s->type() == section_type::BLOCK) {
+          dwarfs::writer::internal::block_compression_info bci;
+          auto catstr = fs.get_block_category(block_no);
+          std::optional<fragment_category::value_type> cat;
+
+          if (catstr) {
+            cat = cat_resolver.category_value(catstr.value());
           }
+
+          writer.check_block_compression(
+              s->compression(), parser->section_data(*s), cat,
+              opts.change_block_size ? &bci : nullptr);
+
+          if (opts.change_block_size) {
+            DWARFS_CHECK(block_no == blocks.size(),
+                         fmt::format("block_no ({}) != blocks.size() ({})",
+                                     block_no, blocks.size()));
+            LOG_DEBUG << "adding block " << block_no
+                      << " uncompressed size: " << bci.uncompressed_size;
+            auto& info = blocks.emplace_back();
+            info.block = block_no;
+            info.uncompressed_size = bci.uncompressed_size;
+            info.section = s;
+            info.category_name = catstr;
+            info.metadata = bci.metadata;
+            info.constraints = bci.constraints;
+          }
+
+          ++block_no;
         }
-        ++block_no;
       }
+
+      tv << "checked compression for " << block_no << " blocks";
+    }
+
+    if (opts.change_block_size) {
+      {
+        auto tv = LOG_TIMED_VERBOSE;
+
+        mapped_blocks =
+            build_block_mappings(blocks, opts.change_block_size.value());
+
+        tv << "mapped " << blocks.size() << " source blocks to "
+           << mapped_blocks.new_to_old.size() << " target blocks";
+      }
+
+      LOG_DEBUG << block_mappings_to_string(mapped_blocks);
     }
   }
 
-  writer.configure_rewrite(parser->filesystem_size(), fs.num_blocks());
+  writer.configure_rewrite(parser->filesystem_size(),
+                           opts.change_block_size
+                               ? mapped_blocks.new_to_old.size()
+                               : fs.num_blocks());
 
   if (auto header = parser->header()) {
     writer.copy_header(*header);
@@ -121,49 +391,77 @@ void rewrite_filesystem(logger& lgr, dwarfs::reader::filesystem_v2 const& fs,
         return false;
       };
 
+  if (opts.change_block_size) {
+    for (auto const& m : mapped_blocks.new_to_old) {
+      std::optional<fragment_category::value_type> cat;
+
+      if (m.category_name) {
+        cat = cat_resolver.category_value(m.category_name.value());
+      }
+
+      writer.rewrite_block(
+          [&] {
+            auto data = malloc_byte_buffer::create_reserve(m.size);
+            for (auto const& c : m.chunks) {
+              auto range =
+                  fs.read_raw_block_data(c.block, c.offset, c.size).get();
+              data.append(range.data(), range.size());
+            }
+            DWARFS_CHECK(data.size() == m.size,
+                         fmt::format("data size {} != expected size {}",
+                                     data.size(), m.size));
+            return std::pair{data.share(), m.metadata};
+          },
+          m.size, cat);
+    }
+  }
+
   parser->rewind();
 
   while (auto s = parser->next_section()) {
     switch (s->type()) {
-    case section_type::BLOCK: {
-      std::optional<fragment_category::value_type> cat;
-      bool recompress_block{opts.recompress_block};
+    case section_type::BLOCK:
+      if (!opts.change_block_size) {
+        std::optional<fragment_category::value_type> cat;
+        bool recompress_block{opts.recompress_block};
 
-      if (recompress_block) {
-        auto catstr = fs.get_block_category(block_no);
+        if (recompress_block) {
+          auto catstr = fs.get_block_category(block_no);
 
-        if (catstr) {
-          cat = cat_resolver.category_value(catstr.value());
+          if (catstr) {
+            cat = cat_resolver.category_value(catstr.value());
 
-          if (!cat) {
-            LOG_ERROR << "unknown category '" << catstr.value()
-                      << "' for block " << block_no;
-          }
+            if (!cat) {
+              LOG_ERROR << "unknown category '" << catstr.value()
+                        << "' for block " << block_no;
+            }
 
-          if (!opts.recompress_categories.empty()) {
-            bool is_in_set{opts.recompress_categories.contains(catstr.value())};
+            if (!opts.recompress_categories.empty()) {
+              bool is_in_set{
+                  opts.recompress_categories.contains(catstr.value())};
 
-            recompress_block =
-                opts.recompress_categories_exclude ? !is_in_set : is_in_set;
+              recompress_block =
+                  opts.recompress_categories_exclude ? !is_in_set : is_in_set;
+            }
           }
         }
+
+        if (recompress_block && from_none_to_none(s, cat)) {
+          recompress_block = false;
+        }
+
+        if (recompress_block) {
+          log_recompress(s, cat);
+
+          writer.write_section(section_type::BLOCK, s->compression(),
+                               parser->section_data(*s), cat);
+        } else {
+          copy_compressed(s, cat);
+        }
+
+        ++block_no;
       }
-
-      if (recompress_block && from_none_to_none(s, cat)) {
-        recompress_block = false;
-      }
-
-      if (recompress_block) {
-        log_recompress(s, cat);
-
-        writer.write_section(section_type::BLOCK, s->compression(),
-                             parser->section_data(*s), cat);
-      } else {
-        copy_compressed(s, cat);
-      }
-
-      ++block_no;
-    } break;
+      break;
 
     case section_type::METADATA_V2_SCHEMA:
     case section_type::METADATA_V2:
@@ -178,6 +476,12 @@ void rewrite_filesystem(logger& lgr, dwarfs::reader::filesystem_v2 const& fs,
           auto builder =
               metadata_builder(lgr, std::move(*md), fsopts.get(), fs.version(),
                                opts.rebuild_metadata.value());
+
+          if (opts.change_block_size) {
+            builder.set_block_size(opts.change_block_size.value());
+            builder.remap_blocks(mapped_blocks.old_to_new);
+          }
+
           auto [schema, data] =
               metadata_freezer(LOG_GET_LOGGER).freeze(builder.build());
 
diff --git a/src/writer/filesystem_writer.cpp b/src/writer/filesystem_writer.cpp
index dd6d6563..f3726b77 100644
--- a/src/writer/filesystem_writer.cpp
+++ b/src/writer/filesystem_writer.cpp
@@ -133,7 +133,7 @@ class fsblock {
           std::shared_ptr<compression_progress> pctx);
 
   fsblock(section_type type, block_compressor const& bc,
-          std::span<uint8_t const> data, compression_type data_comp_type,
+          delayed_data_fn_type data, size_t uncompressed_size,
           std::shared_ptr<compression_progress> pctx);
 
   void
@@ -371,26 +371,27 @@ class compressed_fsblock : public fsblock::impl {
 class rewritten_fsblock : public fsblock::impl {
  public:
   rewritten_fsblock(section_type type, block_compressor const& bc,
-                    std::span<uint8_t const> data,
-                    compression_type data_comp_type,
+                    delayed_data_fn_type data, size_t uncompressed_size,
                     std::shared_ptr<compression_progress> pctx)
       : type_{type}
       , bc_{bc}
-      , data_{data}
+      , data_{std::move(data)}
       , comp_type_{bc_.type()}
       , pctx_{std::move(pctx)}
-      , data_comp_type_{data_comp_type} {
+      , uncompressed_size_{uncompressed_size} {
     DWARFS_CHECK(bc_, "block_compressor must not be null");
   }
 
   void compress(worker_group& wg, std::optional<std::string> meta) override {
+    DWARFS_CHECK(!meta,
+                 "metadata not supported for rewritten_fsblock::compress");
+
     std::promise<void> prom;
     future_ = prom.get_future();
 
-    wg.add_job(
-        [this, prom = std::move(prom), meta = std::move(meta)]() mutable {
-          compress_job(std::move(prom), std::move(meta));
-        });
+    wg.add_job([this, prom = std::move(prom)]() mutable {
+      compress_job(std::move(prom));
+    });
   }
 
   void wait_until_compressed() override { future_.get(); }
@@ -406,7 +407,7 @@ class rewritten_fsblock : public fsblock::impl {
     return block_data_.value().span();
   }
 
-  size_t uncompressed_size() const override { return data_.size(); }
+  size_t uncompressed_size() const override { return uncompressed_size_; }
 
   size_t size() const override {
     std::lock_guard lock(mx_);
@@ -444,31 +445,20 @@ class rewritten_fsblock : public fsblock::impl {
   }
 
  private:
-  void compress_job(std::promise<void> prom, std::optional<std::string> meta) {
+  void compress_job(std::promise<void> prom) {
     try {
-      shared_byte_buffer block;
+      auto [block, meta] = data_();
 
-      {
-        // TODO: we don't have to do this for uncompressed blocks
-        block_decompressor bd(data_comp_type_, data_);
-        block = bd.start_decompression(malloc_byte_buffer::create());
-        bd.decompress_frame(bd.uncompressed_size());
+      pctx_->bytes_in += block.size(); // TODO: data_.size()?
 
-        if (!meta) {
-          meta = bd.metadata();
-        }
-
-        pctx_->bytes_in += block.size(); // TODO: data_.size()?
-
-        try {
-          if (meta) {
-            block = bc_.compress(block, *meta);
-          } else {
-            block = bc_.compress(block);
-          }
-        } catch (bad_compression_ratio_error const&) {
-          comp_type_ = compression_type::NONE;
+      try {
+        if (meta) {
+          block = bc_.compress(block, *meta);
+        } else {
+          block = bc_.compress(block);
         }
+      } catch (bad_compression_ratio_error const&) {
+        comp_type_ = compression_type::NONE;
       }
 
       pctx_->bytes_out += block.size();
@@ -487,14 +477,14 @@ class rewritten_fsblock : public fsblock::impl {
   section_type const type_;
   block_compressor const& bc_;
   mutable std::recursive_mutex mx_;
-  std::span<uint8_t const> data_;
+  delayed_data_fn_type data_;
   std::optional<shared_byte_buffer> block_data_;
   std::future<void> future_;
   std::optional<uint32_t> number_;
   std::optional<section_header_v2> mutable header_;
   compression_type comp_type_;
   std::shared_ptr<compression_progress> pctx_;
-  compression_type const data_comp_type_;
+  size_t const uncompressed_size_;
 };
 
 fsblock::fsblock(section_type type, block_compressor const& bc,
@@ -515,10 +505,10 @@ fsblock::fsblock(fs_section sec, std::span<uint8_t const> data,
                                                  std::move(pctx))) {}
 
 fsblock::fsblock(section_type type, block_compressor const& bc,
-                 std::span<uint8_t const> data, compression_type data_comp_type,
+                 delayed_data_fn_type data, size_t uncompressed_size,
                  std::shared_ptr<compression_progress> pctx)
-    : impl_(std::make_unique<rewritten_fsblock>(type, bc, data, data_comp_type,
-                                                std::move(pctx))) {}
+    : impl_(std::make_unique<rewritten_fsblock>(
+          type, bc, std::move(data), uncompressed_size, std::move(pctx))) {}
 
 void fsblock::build_section_header(section_header_v2& sh,
                                    fsblock::impl const& fsb,
@@ -601,12 +591,15 @@ class filesystem_writer_ final : public filesystem_writer_detail {
   void write_metadata_v2_schema(shared_byte_buffer data) override;
   void write_metadata_v2(shared_byte_buffer data) override;
   void write_history(shared_byte_buffer data) override;
-  void check_block_compression(
-      compression_type compression, std::span<uint8_t const> data,
-      std::optional<fragment_category::value_type> cat) override;
+  void check_block_compression(compression_type compression,
+                               std::span<uint8_t const> data,
+                               std::optional<fragment_category::value_type> cat,
+                               block_compression_info* info) override;
   void write_section(section_type type, compression_type compression,
                      std::span<uint8_t const> data,
                      std::optional<fragment_category::value_type> cat) override;
+  void rewrite_block(delayed_data_fn_type data, size_t uncompressed_size,
+                     std::optional<fragment_category::value_type> cat) override;
   void write_compressed_section(fs_section const& sec,
                                 std::span<uint8_t const> data) override;
   void flush() override;
@@ -624,6 +617,10 @@ class filesystem_writer_ final : public filesystem_writer_detail {
   write_block_impl(fragment_category cat, shared_byte_buffer data,
                    block_compressor const& bc, std::optional<std::string> meta,
                    physical_block_cb_type physical_block_cb);
+  void
+  write_section_delayed_data(section_type type, delayed_data_fn_type data,
+                             size_t uncompressed_size,
+                             std::optional<fragment_category::value_type> cat);
   void on_block_merged(block_holder_type holder);
   void write_section_impl(section_type type, shared_byte_buffer data);
   void write(fsblock const& fsb);
@@ -888,7 +885,8 @@ void filesystem_writer_<LoggerPolicy>::write_section_impl(
 template <typename LoggerPolicy>
 void filesystem_writer_<LoggerPolicy>::check_block_compression(
     compression_type compression, std::span<uint8_t const> data,
-    std::optional<fragment_category::value_type> cat) {
+    std::optional<fragment_category::value_type> cat,
+    block_compression_info* info) {
   block_compressor const* bc{nullptr};
 
   if (cat) {
@@ -897,11 +895,11 @@ void filesystem_writer_<LoggerPolicy>::check_block_compression(
     bc = &default_bc_.value();
   }
 
+  block_decompressor bd(compression, data);
+
   if (auto reqstr = bc->metadata_requirements(); !reqstr.empty()) {
     auto req = compression_metadata_requirements<nlohmann::json>{reqstr};
 
-    block_decompressor bd(compression, data);
-
     try {
       req.check(bd.metadata());
     } catch (std::exception const& e) {
@@ -912,12 +910,19 @@ void filesystem_writer_<LoggerPolicy>::check_block_compression(
       DWARFS_THROW(runtime_error, msg);
     }
   }
+
+  if (info) {
+    info->uncompressed_size = bd.uncompressed_size();
+    info->metadata = bd.metadata();
+    if (info->metadata) {
+      info->constraints = bc->get_compression_constraints(*info->metadata);
+    }
+  }
 }
 
 template <typename LoggerPolicy>
-void filesystem_writer_<LoggerPolicy>::write_section(
-    section_type type, compression_type compression,
-    std::span<uint8_t const> data,
+void filesystem_writer_<LoggerPolicy>::write_section_delayed_data(
+    section_type type, delayed_data_fn_type data, size_t uncompressed_size,
     std::optional<fragment_category::value_type> cat) {
   {
     std::unique_lock lock(mx_);
@@ -933,7 +938,8 @@ void filesystem_writer_<LoggerPolicy>::write_section(
 
     auto& bc = get_compressor(type, cat);
 
-    auto fsb = std::make_unique<fsblock>(type, bc, data, compression, pctx_);
+    auto fsb = std::make_unique<fsblock>(type, bc, std::move(data),
+                                         uncompressed_size, pctx_);
 
     fsb->set_block_no(section_number_++);
     fsb->compress(wg_);
@@ -944,6 +950,32 @@ void filesystem_writer_<LoggerPolicy>::write_section(
   cond_.notify_one();
 }
 
+template <typename LoggerPolicy>
+void filesystem_writer_<LoggerPolicy>::write_section(
+    section_type type, compression_type compression,
+    std::span<uint8_t const> data,
+    std::optional<fragment_category::value_type> cat) {
+  auto bd = block_decompressor(compression, data);
+  auto uncompressed_size = bd.uncompressed_size();
+
+  write_section_delayed_data(
+      type,
+      [bd = std::move(bd)]() mutable {
+        auto block = bd.start_decompression(malloc_byte_buffer::create());
+        bd.decompress_frame(bd.uncompressed_size());
+        return std::pair{std::move(block), bd.metadata()};
+      },
+      uncompressed_size, cat);
+}
+
+template <typename LoggerPolicy>
+void filesystem_writer_<LoggerPolicy>::rewrite_block(
+    delayed_data_fn_type data, size_t uncompressed_size,
+    std::optional<fragment_category::value_type> cat) {
+  write_section_delayed_data(section_type::BLOCK, std::move(data),
+                             uncompressed_size, cat);
+}
+
 template <typename LoggerPolicy>
 void filesystem_writer_<LoggerPolicy>::write_compressed_section(
     fs_section const& sec, std::span<uint8_t const> data) {
diff --git a/src/writer/internal/metadata_builder.cpp b/src/writer/internal/metadata_builder.cpp
index 6949eb2a..e4dc3bc4 100644
--- a/src/writer/internal/metadata_builder.cpp
+++ b/src/writer/internal/metadata_builder.cpp
@@ -135,6 +135,7 @@ class metadata_builder_ final : public metadata_builder::impl {
                       uint32_t num_inodes) override;
 
   void gather_global_entry_data(global_entry_data const& ge_data) override;
+  void remap_blocks(std::span<block_mapping const> mapping) override;
 
   thrift::metadata::metadata const& build() override;
 
@@ -269,6 +270,103 @@ void metadata_builder_<LoggerPolicy>::gather_global_entry_data(
   md_.timestamp_base() = ge_data.get_timestamp_base();
 }
 
+template <typename LoggerPolicy>
+void metadata_builder_<LoggerPolicy>::remap_blocks(
+    std::span<block_mapping const> mapping) {
+  using chunks_t = typename decltype(md_.chunks())::value_type;
+  using chunk_table_t = typename decltype(md_.chunk_table())::value_type;
+  using categories_t = typename decltype(md_.block_categories())::value_type;
+  using category_metadata_t =
+      typename decltype(md_.block_category_metadata())::value_type;
+
+  auto tv = LOG_TIMED_VERBOSE;
+
+  std::span<typename chunks_t::value_type> old_chunks = md_.chunks().value();
+  std::span<typename chunk_table_t::value_type> old_chunk_table =
+      md_.chunk_table().value();
+
+  DWARFS_CHECK(!old_chunk_table.empty(), "chunk table must not be empty");
+
+  chunks_t new_chunks;
+  chunk_table_t new_chunk_table;
+
+  new_chunk_table.push_back(0);
+
+  for (size_t i = 0; i < old_chunk_table.size() - 1; ++i) {
+    auto chunks = old_chunks.subspan(
+        old_chunk_table[i], old_chunk_table[i + 1] - old_chunk_table[i]);
+
+    std::vector<block_chunk> mapped_chunks;
+
+    for (auto const& chunk : chunks) {
+      DWARFS_CHECK(chunk.block().value() < mapping.size(),
+                   "chunk block out of range");
+      auto mapped = mapping[chunk.block().value()].map_chunk(
+          chunk.offset().value(), chunk.size().value());
+      DWARFS_CHECK(!mapped.empty(), "mapped chunk list is empty");
+
+      auto first = mapped.begin();
+
+      if (!mapped_chunks.empty() &&
+          mapped_chunks.back().block == mapped.front().block &&
+          mapped_chunks.back().offset + mapped_chunks.back().size ==
+              mapped.front().offset) {
+        // merge with previous chunk
+        mapped_chunks.back().size += mapped.front().size;
+        ++first;
+      }
+
+      mapped_chunks.insert(mapped_chunks.end(), first, mapped.end());
+    }
+
+    for (auto const& chunk : mapped_chunks) {
+      auto& nc = new_chunks.emplace_back();
+      nc.block() = chunk.block;
+      nc.offset() = chunk.offset;
+      nc.size() = chunk.size;
+    }
+
+    new_chunk_table.push_back(new_chunks.size());
+  }
+
+  auto const& old_categories = md_.block_categories();
+  auto const& old_category_metadata = md_.block_category_metadata();
+
+  if (old_categories.has_value() || old_category_metadata.has_value()) {
+    std::unordered_map<uint32_t, uint32_t> block_map;
+    for (auto const& m : mapping) {
+      for (auto const& c : m.chunks) {
+        block_map[c.block] = m.old_block;
+      }
+    }
+
+    if (old_categories.has_value()) {
+      categories_t new_categories;
+      new_categories.resize(block_map.size());
+      for (auto const& [new_block, old_block] : block_map) {
+        new_categories[new_block] = old_categories.value().at(old_block);
+      }
+      md_.block_categories() = std::move(new_categories);
+    }
+
+    if (old_category_metadata.has_value()) {
+      category_metadata_t new_category_metadata;
+      for (auto const& [new_block, old_block] : block_map) {
+        auto it = old_category_metadata.value().find(old_block);
+        if (it != old_category_metadata.value().end()) {
+          new_category_metadata[new_block] = it->second;
+        }
+      }
+      md_.block_category_metadata() = std::move(new_category_metadata);
+    }
+  }
+
+  md_.chunks() = std::move(new_chunks);
+  md_.chunk_table() = std::move(new_chunk_table);
+
+  tv << "remapping blocks...";
+}
+
 template <typename LoggerPolicy>
 void metadata_builder_<LoggerPolicy>::update_inodes() {
   bool const update_uid{options_.uid.has_value()};
@@ -680,6 +778,33 @@ void metadata_builder_<LoggerPolicy>::upgrade_metadata(
 
 } // namespace
 
+std::vector<block_chunk>
+block_mapping::map_chunk(size_t offset, size_t size) const {
+  std::vector<block_chunk> mapped;
+
+  size_t pos{0};
+
+  for (auto const& chunk : chunks) {
+    if (pos + chunk.size > offset) {
+      auto mapped_offset = offset - pos;
+      auto mapped_size = std::min(size, chunk.size - mapped_offset);
+      mapped.push_back(
+          {chunk.block, chunk.offset + mapped_offset, mapped_size});
+      size -= mapped_size;
+      if (size == 0) {
+        break;
+      }
+      offset += mapped_size;
+    }
+
+    pos += chunk.size;
+  }
+
+  DWARFS_CHECK(size == 0, "failed to map chunk, size mismatch");
+
+  return mapped;
+}
+
 metadata_builder::metadata_builder(logger& lgr, metadata_options const& options)
     : impl_{
           make_unique_logging_object<impl, metadata_builder_, logger_policies>(
diff --git a/tools/src/mkdwarfs_main.cpp b/tools/src/mkdwarfs_main.cpp
index 5fcc1b47..0fad9ca6 100644
--- a/tools/src/mkdwarfs_main.cpp
+++ b/tools/src/mkdwarfs_main.cpp
@@ -420,7 +420,7 @@ int mkdwarfs_main(int argc, sys_char** argv, iolayer const& iol) {
   bool no_progress = false, remove_header = false, no_section_index = false,
        force_overwrite = false, no_history = false,
        no_history_timestamps = false, no_history_command_line = false,
-       rebuild_metadata = false;
+       rebuild_metadata = false, change_block_size = false;
   unsigned level;
   int compress_niceness;
   uint16_t uid, gid;
@@ -531,6 +531,9 @@ int mkdwarfs_main(int argc, sys_char** argv, iolayer const& iol) {
     ("rebuild-metadata",
         po::value<bool>(&rebuild_metadata)->zero_tokens(),
         "fully rebuild metadata")
+    ("change-block-size",
+        po::value<bool>(&change_block_size)->zero_tokens(),
+        "change block size when recompressing")
     ("recompress-categories",
         po::value<std::string>(&recompress_categories),
         "only recompress blocks of these categories")
@@ -890,7 +893,8 @@ int mkdwarfs_main(int argc, sys_char** argv, iolayer const& iol) {
 
   path = iol.os->canonical(path);
 
-  bool recompress = vm.contains("recompress") || rebuild_metadata;
+  bool recompress =
+      vm.contains("recompress") || rebuild_metadata || change_block_size;
   utility::rewrite_options rw_opts;
   if (recompress) {
     std::unordered_map<std::string, unsigned> const modes{
@@ -900,8 +904,12 @@ int mkdwarfs_main(int argc, sys_char** argv, iolayer const& iol) {
         {"none", 0},
     };
 
-    if (recompress_opts.empty() && rebuild_metadata) {
-      recompress_opts = "metadata";
+    if (recompress_opts.empty()) {
+      if (change_block_size) {
+        recompress_opts = "all";
+      } else if (rebuild_metadata) {
+        recompress_opts = "metadata";
+      }
     }
 
     if (auto it = modes.find(recompress_opts); it != modes.end()) {
@@ -913,6 +921,12 @@ int mkdwarfs_main(int argc, sys_char** argv, iolayer const& iol) {
     }
 
     if (!recompress_categories.empty()) {
+      if (change_block_size) {
+        iol.err
+            << "cannot use --recompress-categories with --change-block-size\n";
+        return 1;
+      }
+
       std::string_view input = recompress_categories;
       if (input.front() == '!') {
         rw_opts.recompress_categories_exclude = true;
@@ -1412,9 +1426,12 @@ int mkdwarfs_main(int argc, sys_char** argv, iolayer const& iol) {
 
   try {
     if (recompress) {
-      if (rebuild_metadata) {
+      if (rebuild_metadata || change_block_size) {
         rw_opts.rebuild_metadata = options.metadata;
       }
+      if (change_block_size) {
+        rw_opts.change_block_size = UINT64_C(1) << sf_config.block_size_bits;
+      }
       utility::rewrite_filesystem(lgr, *input_filesystem, *fsw, *cat_resolver,
                                   rw_opts);
     } else {