[OmniOS-discuss] Slow scrub on SSD-only pool

[Stephan Budach]

Am 21.04.16 um 18:36 schrieb Richard Elling:
>> On Apr 21, 2016, at 7:47 AM, Chris Siebenmann <cks at cs.toronto.edu> wrote:
>>
>> [About ZFS scrub tunables:]
>>> Interesting read - and it surely works. If you set the tunable before
>>> you start the scrub you can immediately see the thoughput being much
>>> higher than with the standard setting. [...]
>> It's perhaps worth noting here that the scrub rate shown in 'zpool
>> status' is a cumulative one, ie the average scrub rate since the scrub
>> started. As far as I know the only way to get the current scrub rate is
>> run 'zpool status' twice with some time in between and then look at how
>> much progress the scrub's made during that time.
> Scrub rate measured in IOPS or bandwidth is not useful. Neither is a reflection
> of the work being performed in ZFS nor the drives.
>
>> As such, increasing the scrub speed in the middle of what had been a
>> slow scrub up to that point probably won't make a massive or immediate
>> difference in the reported scrub rate. You should see it rising over
>> time, especially if you drastically speeded it up, but it's not any sort
>> of instant jump.
>>
>> (You can always monitor iostat, but that mixes in other pool IO. There's
>> probably something clever that can be done with DTrace.)
> I've got some dtrace that will show progress. However, it is only marginally
> useful when you've got multiple datasets.
>
>> This may already be obvious and well known to people, but I figured
>> I'd mention it just in case.
> People fret about scrubs and resilvers, when they really shouldn't. In ZFS
> accessing data also checks and does recovery, so anything they regularly
> access will be unaffected by the subsequent scan. Over the years, I've tried
> several ways to approach teaching people about failures and scrubs/resilvers,
> but with limited success: some people just like to be afraid... Hollywood makes
> a lot of money on them :-)
>   -- richard
>
>
No… not afraid, but I actually do think, that I can judge whether or not 
I want to speed scrubs up and trade in some performance for that. As 
long as I can do that, I am fine with it. And the same applies for 
resilvers, I guess. If you need to resilver one half of a mirrored 
zpool, most people will want that to run as fast as feasible, don't they?

Thanks,
Stephan