Re: [gentoo-user] [OT] Using an odd number of drives in ZFS RaidZ

[Robert David]

On Thursday, July 1, 2021 5:01:25 PM CEST antlists wrote:
> On 01/07/2021 14:47, Robert David wrote:
> > Hi Frank,
> >
> >
> >
> > In any of my data arrays I have long time migrated off the RAIDZ to the
> > MIRROR or RAID10. You will find finally that the RAIDZ is slow and not
> > very flexible. Only think you gain is the extra space in constrained
> > array spaces. For RAID10 it is much easier to raise the size, just
> > resilvering to new bigger disks, removing old and expanding. The
> > resilvering speed is magnitude faster.
> >
> > And anyway much easier to recover
> > in cases of failure.
>
> ARE YOU SURE???
>
> The standard mirror does not cope with corruption very well. Lose a disk
> and resilvering is fast. Corrupt the data, and you'll be tearing your
> hair out why things go wrong randomly, with no automated way, even once
> you've realised what's happened, to recover your data other than a
> restore from backup.

Yes I'm sure. What I meant easier is that the pool is much easier to
handle and recover.

For RAIDZ1 the thing you mention for MIRROR is the same, only it is
multiplied with the amount of disks. So if 1 disk fail and you resilver,
then all the remaining disks spinning to populate the spare. If any of
them fails, then you are screwed. RAIDZ2 is better in this space and in
case of 4 disks it is better when it comes to resiliency (for 10 disks
it may not be true), but you lose the flexibility.

Also time to resilver under RAIDZ is much slower, which means longer
time under unprotected pool. It is always needed to decide what workload
you are serving and how precious the data are.

For data like movies RAIDZ1 is enough I think.

Also it is good to check the SMART data time to time to see the amount
of error corrections (some are ok, but highly rising no). Solaris has
FMA for this to kick in spare. Under home environment it is fine to
check it time to time and consider new disk before the old one
completely dies. This reminds me I need to buy new disk to my home NAS :)
(because of the rising corrections).

And finally, always do backups for the data you are about to save. I got
raspberry pi with attached USB JBOD with two disks serving as backup
station. It is not fast to be a real NAS, but to do send/receive of
incremental snapshots it is enough. I automaticly sync there the
datasets that are worth not to lose (photos, documents, etc), many times
these datasets are also the ones that are not such big.
Ideally put this backup station to some remote location (or at least 
different room).

Robert.

>
> > If you really need the additional space, consider adding second jbod
> > with another disks.
>
> That'd be my approach - migrate a load of stuff off onto another disk
> elsewhere, but that's not what the OP wants to do.
>
> > Robert.
>
> Cheers,
> Wol