[OmniOS-discuss] performance penalty of a slow mirror

[Jim Klimov]

On 2013-08-16 23:30, Entfernt wrote:
> On Fri, 16 Aug 2013 11:47:39 +0200
> Jim Klimov <jimklimov at cos.ru> wrote:
>
>> Overall, with reports that USB is not a very reliable connection method,
>> it would IMHO be safer to use the external disk as a separate pool and
>> set up regular replication of snapshots (perhaps with a handler script
>> in the zfs-auto-snap SMF services).
>
> Thanks for the reply.
>
> I'm attracted to the extra-mirror method because it ensures that the copy is always up to date with no extra work.  Backups involving mirrors will always lag behind the live situation.  How about either:
> (1) detaching and re-attaching the USB disk occasionally ?
> (2) occasional scrubbing ?

Technically this occasional attachment should work. Things to look
out for include:

1) Pools may refuse to work if degraded - which includes removal of
a component disk. See "zpool set failmode=continue" as one way to
work around this...

2) It is possible that if you connect the USB disk to some other
system and import it not read-only, this would cause the other
system's ZFS to continue TXG numbering from where it was left when
you disconnected the disk. It is likely that upon reattachment to
the original pool, the system would at least be confused, and likely
require that the disk is wiped and resilvered from scratch. I hope
(but can't vouch) that there wouldn't be any more serious possibly
hidden damage in such case.

3) If possible, do have a mirror remaining inside the box running
at all times (so the USB disk makes it a 3+way mirror), so that
the system is still reliable even without it.

4) Depending on hardware (USB is usually cheap and some corners may
have been cut by manufacturers), the controller in the external disk
enclosure or in your computer may lock up or otherwise misbehave in
work, especially under load (i.e. due to overheating if nothing else)
so scrubs might actually cause some of the problems they'd find.
Not that they are useless or shouldn't be done - just keep an open
mind if there are more CKSUM errors found on the USB disk than on
others... possibly it needs better cooling or cabling or whatnot.

HTH,
//Jim Klimov