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✏️ Editing: zfsslower_example.txt

Demonstrations of zfsslower, the Linux eBPF/bcc version.

zfsslower shows ZFS reads, writes, opens, and fsyncs, slower than a threshold.
It has been written to work on ZFS on Linux (http://zfsonlinux.org). For
example:

# ./zfsslower 
Tracing ZFS operations slower than 10 ms
TIME     COMM           PID    T BYTES   OFF_KB   LAT(ms) FILENAME
06:31:28 dd             25570  W 131072  38784     303.92 data1
06:31:34 dd             25686  W 131072  38784     388.28 data1
06:31:35 dd             25686  W 131072  78720     519.66 data1
06:31:35 dd             25686  W 131072  116992    405.94 data1
06:31:35 dd             25686  W 131072  153600    433.52 data1
06:31:36 dd             25686  W 131072  188672    314.37 data1
06:31:36 dd             25686  W 131072  222336    372.33 data1
06:31:36 dd             25686  W 131072  254592    309.59 data1
06:31:37 dd             25686  W 131072  285440    304.52 data1
06:31:37 dd             25686  W 131072  315008    236.45 data1
06:31:37 dd             25686  W 131072  343424    193.54 data1
06:31:38 dd             25686  W 131072  370560    286.07 data1
06:31:38 dd             25686  W 131072  396672    251.92 data1
[...]

This shows writes to a "data1" file, each taking well over the 10 ms threshold.
the slowest, on the 3rd line of output, reached 519.66 ms for a 128 Kbyte
write by the "dd" command.

This "latency" is measured from when the operation was issued from the VFS
interface to the file system (via the ZFS POSIX layer), to when it completed.
This spans everything: block device I/O (disk I/O), file system CPU cycles,
file system locks, run queue latency, etc. This is a better measure of the
latency suffered by applications reading from the file system than measuring
this down at the block device interface.

Note that this only traces the common file system operations previously
listed: other file system operations (eg, inode operations including
getattr()) are not traced.

A threshold of 0 will trace all operations. Warning: the output will be
verbose, as it will include all file system cache hits.

# ./zfsslower 0
Tracing ZFS operations
TIME     COMM           PID    T BYTES   OFF_KB   LAT(ms) FILENAME
06:36:07 dd             32242  O 0       0           0.01 data1
06:36:07 dd             32242  W 131072  0           0.25 data1
06:36:07 dd             32242  W 131072  128         0.03 data1
06:36:07 dd             32242  W 131072  256         0.04 data1
06:36:07 dd             32242  W 131072  384         0.04 data1
06:36:07 dd             32242  W 131072  512         0.04 data1
06:36:07 dd             32242  W 131072  640         0.03 data1
06:36:07 dd             32242  W 131072  768         0.03 data1
06:36:07 dd             32242  W 131072  896         0.04 data1
06:36:07 dd             32242  W 131072  1024        0.28 data1
06:36:07 dd             32242  W 131072  1152        0.04 data1
06:36:07 dd             32242  W 131072  1280        0.03 data1
[...]
06:36:07 dd             32242  W 131072  13824       0.04 data1
06:36:07 dd             32242  W 131072  13952       0.04 data1
06:36:07 dd             32242  W 131072  14080       0.04 data1
06:36:07 dd             32242  W 131072  14208     398.92 data1
06:36:07 dd             32242  W 131072  14336       0.04 data1
06:36:07 dd             32242  W 131072  14464       0.04 data1
06:36:07 dd             32242  W 131072  15104       0.03 data1
[...]

The output now includes the open operation for this file ("O"), and then the
writes. Most of the writes are very fast, with only an occasional outlier that
is in the hundreds of milliseconds.

Fortunately this is not a real world environment: I setup a zpool on top of a
XFS file system for testing purposes. More debugging using other tools will
explain these outliers: possibly XFS flushing.

Here's a random read workload, and showing operations slower than 1 ms:

# ./zfsslower 1
Tracing ZFS operations slower than 1 ms
TIME     COMM           PID    T BYTES   OFF_KB   LAT(ms) FILENAME
06:47:30 randread.pl    15431  R 8192    97840       1.03 data1
06:47:30 randread.pl    15431  R 8192    416744      1.12 data1
06:47:31 randread.pl    15431  R 8192    228856      1.96 data1
06:47:31 randread.pl    15431  R 8192    452248      1.02 data1
06:47:31 randread.pl    15431  R 8192    315288      5.90 data1
06:47:31 randread.pl    15431  R 8192    752696      1.20 data1
06:47:31 randread.pl    15431  R 8192    481832      1.39 data1
06:47:31 randread.pl    15431  R 8192    673752      1.39 data1
06:47:31 randread.pl    15431  R 8192    691736      1.01 data1
06:47:31 randread.pl    15431  R 8192    694776      1.78 data1
06:47:31 randread.pl    15431  R 8192    403328      3.75 data1
06:47:31 randread.pl    15431  R 8192    567688      1.08 data1
06:47:31 randread.pl    15431  R 8192    694280      1.31 data1
06:47:31 randread.pl    15431  R 8192    669280      1.06 data1
06:47:31 randread.pl    15431  R 8192    426608      1.56 data1
06:47:31 randread.pl    15431  R 8192    42512       1.01 data1
06:47:31 randread.pl    15431  R 8192    22944       1.33 data1
06:47:31 randread.pl    15431  R 8192    427432      1.48 data1
06:47:31 randread.pl    15431  R 8192    261320      1.28 data1
06:47:31 randread.pl    15431  R 8192    132248      1.23 data1
06:47:31 randread.pl    15431  R 8192    96936       1.04 data1
06:47:31 randread.pl    15431  R 8192    482800      2.63 data1
[...]

A -j option will print just the fields (parsable output, csv):

# ./zfsslower -j 1
ENDTIME_us,TASK,PID,TYPE,BYTES,OFFSET_b,LATENCY_us,FILE
252305490911,randread.pl,17922,R,8192,163446784,1156,data1
252305493852,randread.pl,17922,R,8192,321437696,1129,data1
252305498839,randread.pl,17922,R,8192,475152384,1154,data1
252305505515,randread.pl,17922,R,8192,49094656,1082,data1
252305506774,randread.pl,17922,R,8192,470401024,1245,data1
252305509265,randread.pl,17922,R,8192,553246720,2412,data1
252305512365,randread.pl,17922,R,8192,20963328,1093,data1
252305513755,randread.pl,17922,R,8192,304111616,1350,data1
252305583330,randread.pl,17922,R,8192,166174720,1154,data1
252305593913,randread.pl,17922,R,8192,175079424,1241,data1
252305602833,randread.pl,17922,R,8192,305340416,3307,data1
252305608663,randread.pl,17922,R,8192,655958016,2704,data1
252305611212,randread.pl,17922,R,8192,40951808,1033,data1
252305614609,randread.pl,17922,R,8192,318922752,2687,data1
252305623800,randread.pl,17922,R,8192,246734848,2983,data1
252305711125,randread.pl,17922,R,8192,581795840,1091,data1
252305728694,randread.pl,17922,R,8192,710483968,1034,data1
252305762046,randread.pl,17922,R,8192,329367552,1405,data1
252305798215,randread.pl,17922,R,8192,44482560,1030,data1
252305806748,randread.pl,17922,R,8192,660602880,1069,data1
252305826360,randread.pl,17922,R,8192,616144896,2327,data1
[...]

USAGE message:

# ./zfsslower -h
usage: zfsslower [-h] [-j] [-p PID] [min_ms]

Trace common ZFS file operations slower than a threshold

positional arguments:
  min_ms             minimum I/O duration to trace, in ms (default 10)

optional arguments:
  -h, --help         show this help message and exit
  -j, --csv          just print fields: comma-separated values
  -p PID, --pid PID  trace this PID only

examples:
    ./zfsslower             # trace operations slower than 10 ms (default)
    ./zfsslower 1           # trace operations slower than 1 ms
    ./zfsslower -j 1        # ... 1 ms, parsable output (csv)
    ./zfsslower 0           # trace all operations (warning: verbose)
    ./zfsslower -p 185      # trace PID 185 only

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