zfs discuss - Aug 2009 - Question about mirror vdev performance considerations

With four drives A,A,B,B where A is fast access and/or
high-throughput, and B is either slow to seek and/or has slower
transfer speed, what are the implications for mirrored ZFS pools?

In particular I am wondering how the IO performance will compare between:

zpool create mypool mirror A A mirror B B

and

zpool create mypool mirror A B mirror A B

and

zpool create mypool mirror A B mirror B A

Thanks,
Charles Menser

Your example is too simple :-)

On Aug 12, 2009, at 1:03 PM, Charles Menser wrote:
> With four drives A,A,B,B where A is fast access and/or
> high-throughput, and B is either slow to seek and/or has slower
> transfer speed, what are the implications for mirrored ZFS pools?
>
> In particular I am wondering how the IO performance will compare  
> between:
>
> zpool create mypool mirror A A mirror B B
>
> and
>
> zpool create mypool mirror A B mirror A B
>
> and
>
> zpool create mypool mirror A B mirror B A
Largely the same.  Think of it this way, the expected average latency
is a function of the probability (p) that an IOP would be serviced by  
the
fast disk.  Since you only have two types of disks and one RAID
configuration, the probability of an IOP being satisfied by a fast
disk is 50% any way you slice it.
   expected_latency = p * fast_latency + (p-1) * slow_latency

You''d need something more complex to change the expected
latency equation.

NB, ZFS does not currently schedule IOPS based on a preferred
side.  It may be relatively easy to implement, at least for a directed
preference, but I suspect it is low enough on the priority list that the
fastest way to get there is via the community.
  -- richard