[OmniOS-discuss] performance penalty of a slow mirror

[Jim Klimov]

On 2013-08-15 09:01, Michael Mounteney wrote:
> Hello, as my current OmniOS server doesn't have removable disks, I've
> added an external USB2 HDD as a mirror, so that in the event of fire or
> holiday, the external disk can be taken away as a backup.
>
> How does OmniOS deal with a very slow mirror ?  Does it perform any
> measurements to work out that it should always write to the other
> mirror first, or does it do a 'fair sharing' or writes ?
>
> This is a SuperMicro 5017C-LF
> (http://www.supermicro.com/products/system/1u/5017/sys-5017c-lf.cfm )
> with 2 x 500 GiB 2.5" drives mirrored internally, plus the external
> mirror.  The machine has only USB-2 sockets, not USB-3.

AFAIK, at least the general illumos ZFS so far should not have special
precautions for this (recently discussed as a case for remote-networked
mirrors - that WAN copies should only receive writes and not a fairly
shared half of reads). It may be that lags would cause "degradation"
of the device in pool terms, but so far likely not balancing of IOs any
other than by filling up the request queue. The latter might cause the
slow device to process less requests per second than the faster device,
but the requests that do get scheduled to this slower disk would suffer
a higher latency.

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).

HTH,
//Jim