How to speed up slow zpool scrub?

[Miroslav Lachman]

Adam Nowacki wrote on 04/27/2016 19:36:
> On 2016-04-27 14:05, Miroslav Lachman wrote:

>> Thank you for output of your zpool scrub. It is definitely faster than
>> mine.
>>
>> To: Paul pk1048
>> Mine scrub does not repair anything. Drives are OK (in SMART).
>> CPU is about 70%-90% idle during scrub + rsync backup and drives are
>> about 60%-70% busy according to iostat:
>>
>> root at kiwi ~/# iostat -x -w 10 ada0 ada1 ada2 ada3

>> device     r/s   w/s    kr/s    kw/s qlen svc_t  %b
>> ada0     121.8  16.6   602.1   526.9    3   9.2  52
>> ada1     122.2  16.5   606.9   528.5    4   9.8  54
>> ada2     117.0  14.6   601.7   524.9    2  11.3  60
>> ada3     120.6  13.5   610.1   491.3    0  11.4  61
>>
>> I really don't know why it cannot go faster if nothing is loaded for 100%.
>
> 1) zpool scrub is single threaded with prefetch,

Hmm, this can be the cause. Does it mean that ZFS is faster on CPU with 
higher "per core" power and number of cores (threads) is not so 
important? (in this case of scrub)

> 2) some data blocks do not span all disks (metadata, small files,
> compression)
> End result is that zfs can't always read from all disks during scrub so
> disk utilization is going to be less than 100% even when going at full
> speed.

Thank you for the explanation.

Miroslav Lachman