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180 lines
9.3 KiB
180 lines
9.3 KiB
Demonstrations of btrfsdist, the Linux eBPF/bcc version.
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btrfsdist traces btrfs reads, writes, opens, and fsyncs, and summarizes their
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latency as a power-of-2 histogram. For example:
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# ./btrfsdist
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Tracing btrfs operation latency... Hit Ctrl-C to end.
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^C
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operation = 'read'
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usecs : count distribution
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0 -> 1 : 15 | |
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2 -> 3 : 1308 |******* |
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4 -> 7 : 198 |* |
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8 -> 15 : 0 | |
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16 -> 31 : 11 | |
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32 -> 63 : 361 |* |
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64 -> 127 : 55 | |
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128 -> 255 : 104 | |
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256 -> 511 : 7312 |****************************************|
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512 -> 1023 : 387 |** |
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1024 -> 2047 : 10 | |
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2048 -> 4095 : 4 | |
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operation = 'write'
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usecs : count distribution
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0 -> 1 : 0 | |
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2 -> 3 : 0 | |
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4 -> 7 : 0 | |
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8 -> 15 : 4 |****************************************|
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operation = 'open'
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usecs : count distribution
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0 -> 1 : 1 |********** |
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2 -> 3 : 4 |****************************************|
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This output shows a bi-modal distribution for read latency, with a faster
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mode of 1,308 reads that took between 2 and 3 microseconds, and a slower
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mode of over 7,312 reads that took between 256 and 511 microseconds. It's
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likely that the faster mode was a hit from the in-memory file system cache,
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and the slower mode is a read from a storage device (disk).
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This "latency" is measured from when the operation was issued from the VFS
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interface to the file system, to when it completed. This spans everything:
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block device I/O (disk I/O), file system CPU cycles, file system locks, run
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queue latency, etc. This is a better measure of the latency suffered by
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applications reading from the file system than measuring this down at the
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block device interface.
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Note that this only traces the common file system operations previously
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listed: other file system operations (eg, inode operations including
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getattr()) are not traced.
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An optional interval and a count can be provided, as well as -m to show the
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distributions in milliseconds. For example, two second summaries, five times:
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# ./btrfsdist 2 5
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Tracing btrfs operation latency... Hit Ctrl-C to end.
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03:40:49:
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operation = 'read'
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usecs : count distribution
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0 -> 1 : 15 | |
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2 -> 3 : 833 |******** |
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4 -> 7 : 127 |* |
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8 -> 15 : 0 | |
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16 -> 31 : 8 | |
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32 -> 63 : 907 |******** |
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64 -> 127 : 91 | |
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128 -> 255 : 246 |** |
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256 -> 511 : 4164 |****************************************|
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512 -> 1023 : 193 |* |
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1024 -> 2047 : 4 | |
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2048 -> 4095 : 6 | |
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4096 -> 8191 : 2 | |
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03:40:51:
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operation = 'read'
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usecs : count distribution
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0 -> 1 : 25 | |
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2 -> 3 : 1491 |*************** |
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4 -> 7 : 218 |** |
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8 -> 15 : 0 | |
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16 -> 31 : 16 | |
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32 -> 63 : 1527 |*************** |
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64 -> 127 : 319 |*** |
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128 -> 255 : 429 |**** |
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256 -> 511 : 3841 |****************************************|
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512 -> 1023 : 232 |** |
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1024 -> 2047 : 3 | |
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2048 -> 4095 : 6 | |
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4096 -> 8191 : 1 | |
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8192 -> 16383 : 1 | |
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03:40:53:
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operation = 'read'
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usecs : count distribution
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0 -> 1 : 27 | |
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2 -> 3 : 2999 |********************************* |
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4 -> 7 : 407 |**** |
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8 -> 15 : 0 | |
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16 -> 31 : 46 | |
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32 -> 63 : 3538 |****************************************|
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64 -> 127 : 595 |****** |
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128 -> 255 : 621 |******* |
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256 -> 511 : 3532 |*************************************** |
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512 -> 1023 : 212 |** |
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1024 -> 2047 : 1 | |
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2048 -> 4095 : 0 | |
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4096 -> 8191 : 0 | |
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8192 -> 16383 : 0 | |
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16384 -> 32767 : 1 | |
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03:40:55:
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operation = 'read'
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usecs : count distribution
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0 -> 1 : 221 | |
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2 -> 3 : 12580 |****************************************|
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4 -> 7 : 1366 |**** |
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8 -> 15 : 0 | |
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16 -> 31 : 289 | |
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32 -> 63 : 10782 |********************************** |
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64 -> 127 : 1232 |*** |
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128 -> 255 : 807 |** |
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256 -> 511 : 2299 |******* |
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512 -> 1023 : 135 | |
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1024 -> 2047 : 5 | |
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2048 -> 4095 : 2 | |
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03:40:57:
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operation = 'read'
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usecs : count distribution
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0 -> 1 : 73951 |************************* |
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2 -> 3 : 117639 |****************************************|
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4 -> 7 : 7943 |** |
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8 -> 15 : 1841 | |
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16 -> 31 : 1143 | |
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32 -> 63 : 5006 |* |
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64 -> 127 : 483 | |
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128 -> 255 : 242 | |
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256 -> 511 : 253 | |
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512 -> 1023 : 84 | |
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1024 -> 2047 : 23 | |
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This shows a read workload that begins bimodal, and eventually the second
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mode disappears. The reason for this is that the workload cached during
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tracing. Note that the rate also increased, with over 200k reads for the
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final two second sample.
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USAGE message:
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# ./btrfsdist -h
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usage: btrfsdist [-h] [-T] [-m] [-p PID] [interval] [count]
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Summarize btrfs operation latency
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positional arguments:
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interval output interval, in seconds
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count number of outputs
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optional arguments:
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-h, --help show this help message and exit
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-T, --notimestamp don't include timestamp on interval output
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-m, --milliseconds output in milliseconds
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-p PID, --pid PID trace this PID only
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examples:
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./btrfsdist # show operation latency as a histogram
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./btrfsdist -p 181 # trace PID 181 only
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./btrfsdist 1 10 # print 1 second summaries, 10 times
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./btrfsdist -m 5 # 5s summaries, milliseconds
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