Roch - PAE wrote:
Thinking some more about this. If your requirements does
mandate some form of mirroring, then it truly seems that ZFS
should take that in charge if only because of the
self-healing characteristics. So I feel the storage array's
job is to export low latency Luns to ZFS.
The hard part is getting a set of simple requirements. As you go into
more complex data center environments you get hit with older Solaris
revs, other OSs, SOX compliance issues, etc. etc. etc. The world where
most of us seem to be playing with ZFS is on the lower end of the
complexity scale. Sure, throw your desktop some fast SATA drives. No
problem. Oh wait, you've got ten Oracle DBs on three E25Ks that need to
be backed up every other blue moon ...
I agree with the general idea that an array, be it one disk or some raid
combination, should simply export low latency LUNs. However, its the
features offered by the array - Like site to site replication - used to
meet more complex requirements that literally slow things down. In many
cases you'll see years old operational procedures causing those low
latency LUNs to slow down even more. Something really hard to get a
customer to undo because a new fangled file system is out. ;)
I'd be happy to live with those simple Luns but I guess some
storage will just refuse to export non-protected luns. Now
we can definitively take advantage of the Array's capability
of exporting highly resilient Luns; RAID-5 seems to fit the
bill rather well here. Even an 9+1 luns will be quite
resilient and have a low block overhead.
I think 99x0 used to do 3+1 only. Now it's 7+1 if I recall. Close enough
I suppose.
So we benefit from the arrays resiliency as well as it's low
latency characteristics. And we mirror data at the ZFS level
which means great performance and great data integrity and
great availability.
Note that ZFS write characteristics (all sequential) means
that we will commonly be filling full stripes on the luns
thus avoiding the partial stripe performance pitfall.
One thing comes to mind in that case. Many arrays do sequential detect
on the blocks that come in to the front end ports.
If things get split up to much or out of order or <insert some strange
array characteristic here> then you could induce more latency as the
array does cartwheels trying to figure out whats going on.
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