How to speed up slow zpool scrub?

[PK1048]

> On Apr 26, 2016, at 18:27, Andy Farkas <andyf at andyit.com.au> wrote:
> 
> On 27/04/2016 02:34, Miroslav Lachman wrote:
>> DH wrote on 04/26/2016 17:47:
>>>> 5GB of RAM
>>> 
>>> That seems to be an insufficient amount of system ram when employing zfs.
>>> 
>>> Take a look at this:
>>> 
>>> http://doc.freenas.org/9.3/freenas_intro.html#ram
>> 
>> I know 5GB is not much these days but is memory used for scrubbing a lot? Because I am satisfied with working performance. The only concern is slow scrubbing and I am not sure that more memory helps in this case.

I don’t expect memory to make a big difference in scrub or resilver performance. Rememeber the way ZFS uses memory is as both write buffer and read cache (all in the ARC). So insufficient memory will hurt real performance but it should not have any real effect on a scrub or resilver (which are both operations that read all the data that has been written to the zpool and check it against the metadata for consistency).

Scrubs (and resilver) operations are essentially all random I/O. Those drives are low end, low performance, desktop drives.

The fact that the scrub _repaired_ anything means that there was damage to data. If all of the data on the drives is good, then a scrub has nothing to repair.

What does an `iostat -x 1` show during the scrub ? How about a `zpool iostat -v 1` ? How hard are you hitting those drives and are they all really good ? I have seen svc_t values differ by a factor of two among drives of all the same make and model. Is one drive slower than the rest ? Perhaps that drive is on the way out.