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108 lines
5.4 KiB
Plaintext
108 lines
5.4 KiB
Plaintext
The following is a demonstration of the diskhits command.
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Here we run diskhits on a large file, /extra1/contents with is 46 Mb, and
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currently hasn't been accessed (so isn't in any cache).
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While diskhits is running, the file is grep'd in another window. This causes
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the entire file to be read,
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# ./diskhits /extra1/contents
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Tracing... Hit Ctrl-C to end.
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^C
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Location (KB),
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value ------------- Distribution ------------- count
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< 0 | 0
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0 |@@ 47
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2303 |@@ 41
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4606 |@@ 41
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6909 |@@ 42
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9212 |@@ 41
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11515 |@@ 41
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13818 |@@ 42
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16121 |@@ 43
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18424 |@@ 42
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20727 |@@ 41
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23030 |@@ 41
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25333 |@@ 41
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27636 |@@ 41
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29939 |@@ 42
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32242 |@@ 44
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34545 |@@ 41
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36848 |@@ 41
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39151 |@@ 41
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41454 |@@ 41
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43757 |@@ 40
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>= 46060 | 0
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Size (KB),
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value ------------- Distribution ------------- count
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4 | 0
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8 | 6
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16 | 10
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32 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 818
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64 | 0
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Total RW: 46064 KB
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Ok, so the file was read evently with each access around 32 to 63 Kb in size,
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and a total of 46 Mb read. This all makes sense, as it is reading the file
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for the first time.
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Now the same file is grep'd with diskhits running, this time we can see what
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effect caching the file has made,
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# ./diskhits /extra1/contents
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Tracing... Hit Ctrl-C to end.
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^C
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Location (KB),
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value ------------- Distribution ------------- count
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2303 | 0
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4606 | 5
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6909 |@ 67
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9212 |@@@@ 170
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11515 |@@@@@ 216
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13818 |@@@@@ 224
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16121 |@@@@@@ 287
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18424 |@@@@@ 227
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20727 |@@@ 144
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23030 |@@ 75
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25333 |@ 59
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27636 |@ 42
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29939 |@ 41
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32242 |@ 44
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34545 |@ 41
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36848 |@ 41
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39151 |@ 41
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41454 |@ 41
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43757 |@ 39
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>= 46060 | 0
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Size (KB),
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value ------------- Distribution ------------- count
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2 | 0
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4 |@@@@@@@@@@@@@@@@@@@@@@@@@ 1137
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8 |@@@@@ 211
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16 |@@ 111
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32 |@@@@@@@@ 345
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64 | 0
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Total RW: 29392 KB
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The difference is dramatic. This time only 29 Mb is read, leaving around
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17 Mb that was read from the cache. The way the file is read differs -
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in the later half of the file it looks the same, but in the first half there
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are many more events; oddly enough, this is because the early part of the
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file is cached more, the extra events are likely to be much smaller in size -
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as indicated in the difference in the size distribution.
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It appears that everything less that 4606 Kb has remained in the cache, with
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zero hits for that range.
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