Using the fileops read Probe
Example 11-8 Displaying Read Latencies With the read Probe
You can use the read probe to observe read latencies on different file system types. For example:
fileops:::read
{
@[args[0]->fi_fs] = quantize(args[1]);
}
Running the example script on a server for several seconds generates an output similar to the following example:
# dtrace -s ./read-latency-by-fs.d
dtrace: script './read-latency-by-fs.d' matched 1 probe
^C
tmpfs
value ------------- Distribution ------------- count
64 | 0
128 |@@@@@@@@@ 2
256 |@@@@ 1
512 | 0
1024 | 0
2048 | 0
4096 | 0
8192 |@@@@@@@@@@@@@@@@@@ 4
16384 |@@@@@@@@@ 2
32768 | 0
procfs
value ------------- Distribution ------------- count
1024 | 0
2048 |@@@@@@@ 2
4096 | 0
8192 |@@@@@@@@@@@@@@@@@@ 5
16384 |@@@@@@@@@@@@@@@ 4
32768 | 0
nfs3
value ------------- Distribution ------------- count
128 | 0
256 |@@@@ 6
512 |@@@ 5
1024 |@@@@@ 7
2048 |@@@@@@@@@@@@@@ 21
4096 |@@@@@@@ 10
8192 |@@@@@@@@ 12
16384 | 0
sockfs
value ------------- Distribution ------------- count
128 | 0
256 |@@@@@ 16
512 |@@@@@@@@ 27
1024 |@@@@@@@ 24
2048 |@@@@@@@@@@@ 38
4096 |@@ 8
8192 |@@@@@@@ 23
16384 | 1
32768 | 0
65536 | 0
131072 | 0
262144 | 0
524288 | 0
1048576 | 0
2097152 | 1
4194304 | 0
fifofs
value ------------- Distribution ------------- count
32 | 0
64 | 1
128 |@@@ 8
256 |@@@@@ 13
512 |@@ 6
1024 |@@ 4
2048 |@@ 4
4096 |@@@@@@@ 18
8192 |@ 2
16384 |@@@ 8
32768 | 1
65536 |@ 3
131072 | 0
262144 | 0
524288 |@ 2
1048576 | 1
2097152 |@@ 6
4194304 |@@@@@@@ 17
8388608 |@ 2
16777216 | 0
33554432 | 1
67108864 | 1
134217728 | 0
zfs
value ------------- Distribution ------------- count
256 | 0
512 |@ 85
1024 |@@ 275
2048 |@ 116
4096 |@@@@@@ 982
8192 |@@@@@@@@@@@@ 1963
16384 | 72
32768 | 34
65536 | 2
131072 | 39
262144 | 3
524288 | 0
1048576 | 53
2097152 |@@ 351
4194304 |@@@@@@@@@ 1360
8388608 |@@@@@@ 1011
16777216 | 18
33554432 | 6
67108864 | 2
134217728 | 3
268435456 | 2
536870912 | 0
In this example, you can see that memory-resident file systems such as tmpfs and pseudo-file systems such as procfs satisfy read requests very quickly. You can also see that file systems such as zfs have a bi-modal distribution. Reads that are satisfied from the in-memory cache happen very quickly and the reads that must go to disk are orders of magnitude slower.
Example 11-9 Displaying the Distribution of Write Sizes With the write Probe
You can use the write probe to observe the distribution of write sizes on the system, independent of file system type. For example:
fileops:::write
{
@ = quantize(args[4]);
}
Running this briefly on a small server generates an output similar to the following:
# dtrace -s ./write-size.d
dtrace: script './write-size.d' matched 1 probe
^C
value ------------- Distribution ------------- count
-1 | 0
0 | 2
1 |@@@@@@@@@@ 190
2 | 5
4 |@ 16
8 |@ 20
16 |@ 24
32 |@@@@ 80
64 |@@@@@@@@@@@@@ 253
128 |@@@@@ 96
256 |@@ 48
512 | 3
1024 |@@ 34
2048 | 2
4096 | 0
8192 | 3
16384 | 0