[CentOS] ZFS on Linux in production?

Fri Oct 25 23:29:02 UTC 2013
Warren Young <warren at etr-usa.com>

On re-reading, I realized I didn't complete some of my thoughts:

On 10/25/2013 00:18, Warren Young wrote:
> ZFS is nicer in this regard, in that it lets you schedule the scrub
> operation.  You can obviously schedule one for btrfs,

...with cron...

> but that doesn't take into account scrub time.

This is important because a ZFS scrub takes absolute lowest priority. 
(Presumably true for btrfs, too.)  Any time the filesystem has to 
service an I/O request, the scrub stops, then resumes when the I/O 
request has completed, unless another has arrived in the meantime.

This means that you cannot know how long a scrub will take unless you 
can exactly predict your future disk I/O.  Scheduling a scrub with cron 
could land you in a situation where the previous scrub is still running 
due to unusually high I/O when another scrub request comes in.

I initially set our ZFS file server up so that it would start scrubbing 
at close of business on Friday, but due to the way ZFS scrub scheduling 
works, the most recent scrub started late Wednesday and ran into 
Thursday.  This isn't a problem.  The scrub doesn't run in parallel to 
normal I/O, I don't even notice that the array is scrubbing itself 
unless I go over and watchen das blinkenlights astaunished.