Hi,
Just wanted to throw in that we discussed
with David Midgett (while waiting for the
next SnapMirror schedule to be triggered)
how he was using SnapMirror. And heard
(correct me if I'm wrong David) that his
copy of data was being accessed by other
clients in read-only fashion effectively
balancing the client load across the CFO
pair. So not only does David have a (local)
disaster recovery copy of his data, but its
also accessible (in read-only mode) at all
times (after the baseline transfer has completed).
Cheers,
Grant
-----Original Message-----
From: alexei(a)mindspring.net [mailto:alexei@mindspring.net]
Sent: Thursday, July 29, 1999 3:35 PM
To: Sam Schorr
Cc: toasters(a)mathworks.com
Subject: Re: SnapMirror Very Slow
Sam Schorr <sschorr(a)homestead.com> writes:
> Actually, there is a very good reason to do both. Clustering only protects
> filer head failures. If a volume on a filer loses 2 disks in 1 RAID group,
> the volume built on that RAID group is useless and needs to be rebuilt upon
> disk repair. So, by using SNAPMIRROR to "almost realtime" copy data to a
> read-only volume on another filer, upon disk failure that volume can be made
> "live" or the master volume. This is a strategy we are employing to fully
> protect our users' data and insure the highest availability of that data.
I am not arguing that doing both is not (potentially) good,
but doing both on the same set of filers is troublesome. Individually,
each solution addresses a set of problems, and each has its own set of problems.
With CF (cluster failover), you get transparent (or close to) failover such that
a client does not percieve a failure, just a pause. The cluster partner is then
serving both sets of data (its own and that of its partner). If you do not
size the filer correctly, you are going to be in trouble, especially if you
loose
a disk during failover. Snapshots are an additional overhead.
With snapmirror, you have a copy of the data N minutes old (where N is a
function
based on the network latency, the size of the volume (clustermap overhead), size
of the network pipe and ammount of deltas). In order to make that copy "live",
you will need to update mount points across your clients in order to point at
the now-read-write snapmirror volume on the other filer.
So, doing snapmirror between cluster members is not a clear win. You are
better off snapmirroring to a filer that is not part of the cluster. That way
you can isolate your disaster backup copy from any potential problems the
cluster might experience.
Alex