VFS(), Virtual file system is where applications interact with the underlying storage system. Hence, the responsiveness of an application depends more on latency of this layer than the actual latency of the underlying disks.
Just for curiosity, I have measured the VFS read latency before, during and after scrub using the following dtrace script.
The system tested was an E3-1230 with 16GB RAM, 6 disks in a raidz2 pool, 1 256GB L2ARC and 1 5GB SLOG.
Before scrub, the latencies in ns
During scrub,
After scrub,
Before and after scrub, there were just a number of cases in which latency went higher than 10 ms. Still, it may make an application feels slow. However, during scrub, the maximum latency could reach as high as 1 s, which would surely affect responsiveness of some applications.
Just for curiosity, I have measured the VFS read latency before, during and after scrub using the following dtrace script.
Code:
vfs::vop_read:entry
{
self->st[stackdepth] = timestamp;
}
vfs::vop_read:return
/self->st[stackdepth]/
{
@ = quantize(timestamp - self->st[stackdepth]);
self->st[stackdepth] = 0;
}
tick-1200s
{
exit(0);
}The system tested was an E3-1230 with 16GB RAM, 6 disks in a raidz2 pool, 1 256GB L2ARC and 1 5GB SLOG.
Before scrub, the latencies in ns
Code:
value ------------- Distribution ------------- count
512 | 0
1024 | 84
2048 | 950
4096 | 893
8192 |@@@@ 19073
16384 |@@ 10824
32768 |@@@@@@@@@@@@@@@@@@@@@ 93553
65536 |@@@@@@@@ 36897
131072 |@ 3445
262144 | 1480
524288 |@@ 10453
1048576 |@ 2747
2097152 | 695
4194304 | 288
8388608 | 120
16777216 | 55
33554432 | 4
67108864 | 0During scrub,
Code:
value ------------- Distribution ------------- count
256 | 0
512 | 10
1024 | 482
2048 |@ 3718
4096 |@@@@ 30482
8192 |@@@@@@@@@@@@@@@@@@ 128855
16384 |@@@@@@@@@ 63613
32768 |@@@@@@ 41409
65536 |@ 6978
131072 | 2065
262144 |@ 5404
524288 |@ 9339
1048576 | 1107
2097152 | 472
4194304 | 35
8388608 | 3
16777216 | 3
33554432 | 10
67108864 | 14
[U] 134217728 | 6
268435456 | 6
536870912 | 2
1073741824 | 0[/U]After scrub,
Code:
value ------------- Distribution ------------- count
512 | 0
1024 | 121
2048 | 877
4096 |@@ 11277
8192 |@@@@@@ 41507
16384 |@@@@ 26238
32768 |@@@@@@@@@@@@@@@@@@@@ 136957
65536 |@@@@@@ 39477
131072 |@ 3511
262144 | 2129
524288 |@@ 12421
1048576 | 1805
2097152 | 635
4194304 | 125
8388608 | 40
16777216 | 27
33554432 | 1
67108864 | 3
134217728 | 0Before and after scrub, there were just a number of cases in which latency went higher than 10 ms. Still, it may make an application feels slow. However, during scrub, the maximum latency could reach as high as 1 s, which would surely affect responsiveness of some applications.