We had another case where a pool scrub was taking a very long time this past weekend and week; on a 3 TB pool, 'zpool status' was reporting ongoing scrub rates of under 3 MB/s. This got us to go on some Internet searches for kernel tunables that might be able to speed this up. The results proved to be extremely interesting. I will cut to the punchline: with one change we got the pool scrubbing at roughly 100 Mbytes/second, which is the maximum scrub IO rate a fileserver can maintain at the moment. Also, it turns out that when I blithely asserted that our scrubs were being killed by having to do random IO I was almost certainly dead wrong.

(One reason we were willing to try changing tunable parameters on a live production system was that this pool was scrubbing so disastrously slow that we were seriously worried about resilver times for it if it ever needed a disk replacement.)

The two good references we immediately found for tuning ZFS scrubs and resilvers are this serverfault question and answer and ZFS: Performance Tuning for Scrubs and Resilvers. Rather than change all of their recommended parameters at once, I opted to make one change at a time and observe the effects (just in case a change caused the server to choke). The first change I made was to set zfs_scrub_delay to 0; this immediately accelerated the scrub rate to 100 Mbytes/sec.

Let's start with a quote from the code in dsl_scan.c:

int zfs_scrub_delay = 4;     /* number of ticks to delay scrub */
int zfs_scan_idle = 50;      /* idle window in clock ticks */

How these variables are used is that every time a ZFS scrub is about to issue a read request, it checks to see if some normal read or write IO has happened within zfs_scan_idle ticks. If it has, it delays zfs_scrub_delay ticks before issuing the IO or doing anything else. If your pool is sufficiently busy to hit this condition more or less all of the time, ZFS scrubs will only be able to make at most a relatively low number of reads a second; if HZ is how many ticks in a second, the issue rate is HZ / 4 by default. In standard OmniOS kernels, HZ is almost always 100; that is, there are 100 ticks a second. If your regular pool users are churning around enough to do one actual IO every half a second, your scrubs are clamped to no more than 25 reads a second. If each read is for a full 128 KB ZFS block, that's a scrub rate of about 3.2 MBytes/sec at most (and there are other things that can reduce it, too).

Setting zfs_scrub_delay to 0 eliminates this clamping of scrub reads in the face of other IO; instead your scrub is on much more equal footing with active user IO. Unfortunately you cannot set it to any non-zero value lower than 1 tick, and 1 tick will clamp you to 100 reads a second, which is probably not fast enough for many people.

This does not eliminate slowness due to scrubs (and resilvers) potentially having to do a lot of random reads, so it will not necessarily eliminate all of your scrub speed problems. But if you have a pool that seems to scrub at a frustratingly variable speed, sometimes scrubbing in a day and sometimes taking all week, you are probably running into ZFS scrubs backing off in the face of other IO and it's worth exploring this tunable and the others in those links.

On the other tunables, I believe that it's relatively harmless and even useful to tune up zfs_scan_min_time_ms, zfs_resilver_min_time_ms, and zfs_top_maxinflight. Certainly I saw no problems on our server when I set zfs_scan_min_time_ms to 5000 and increased zfs_top_maxinflight. However I can't say for sure that it's useful, as our scrub rate had already hit its maximum rate from just the zfs_scrub_delay change.

(And I'm still reading the current Illumos ZFS kernel code to try to understand what these additional tunables really do and mean.)

Sidebar: How to examine and set these tunable variables

To change kernel tunables like this, you need to use 'mdb -kw' to enable writing to things. To see their value, I recommend using '::print', eg:

> zfs_scrub_delay ::print -d
4
> zfs_scan_idle ::print -d
0t50

To set the value, you should to use /W, not the /w that ZFS: Performance Tuning for Scrubs and Resilvers says. The w modifier is for 2-byte shorts, not 4-byte ints, and all of these variables are 4-byte ints (as you can see with '::print -t' and '::sizeof' if you want). A typical example is:

> zfs_scrub_delay/W0
zfs_scrub_delay:0x4 = 0x0

The /W stuff accepts decimal numbers as '0tNNNN' (as '::print -d' shows them, unsurprisingly), so you can do things like:

> zfs_scan_min_time_ms/W0t5000
zfs_scan_min_time_ms: 0x3e8 = 0x1388

(Using '/w' will work on the x86 because the x86 is a little-endian architecture, but please don't get into the habit of doing that. My person view is that if you're going to be poking values into kernel memory it's very much worth being careful about doing it right.)