Richard,
Having read your blog regarding the copies feature, do you have an
opinion on whether mirroring or copies are better for a SAN situation?
It strikes me that since we''re discussing SAN and not local physical
disk, that for a system needing 100GB of usable storage (size chosen
for round numbers) the configuration choices present an interesting
discussion. I would probably throw out raidz(2) right off the top
leaving the following choices between the SAN presenting one 200GB
LUN, two 100GB LUNs, or four 50GB LUNs for the respective configs of
copies=2, raid 0, and raid 0+1. My assumption would be that the 0+1
would still be the best all around solution to balance speed and
redundancy. It would also give you more flexibility moving forward by
adding more vdev mirrors for future space requirements.
Setting copies = X does not seem to change the idea that you''re
storage will cost X times as much as a plain UFS (or ZFS) arrangement
which relies on the hardware for redundancy. Based on your diagrams
in the blog, it would also seem to be slower by a factor of 1/X.
Maybe cost and speed aren''t a factor, but as I often hear "Cheap,
Fast, Reliable, pick two." and being that we''re looking at SAN, I
would think the choice is Fast and Reliable.
Do you have the time/hardware to examine the same things you look
have blogged about before using ZFS+JBOD, now instead using ZFS+SAN?
> Date: Tue, 02 Oct 2007 17:12:06 -0700
> From: Richard Elling <Richard.Elling at Sun.COM>
> Subject: Re: [zfs-discuss] zfs in san
> To: Todd Sawyers <toddsawyers at gmail.com>
> Cc: zfs-discuss at opensolaris.org
> Message-ID: <4702DE56.6080807 at sun.com>
> Content-Type: text/plain; format=flowed; charset=ISO-8859-1
>
> Todd Sawyers wrote:
> > I am planning to use zfs with fiber attached san disk from a emc
symmetrix''s
> > Based on a note in the admin guide it appears that even though
> > the symmetrixs will handle the hardware raid it is still advisable to
create
> > a zfs mirror on the host side to take full advantage of zfs''s
self
> > healing/error checking and correcting.
> >
> > is this true ?
>
> Yes, though you have more options than just mirroring. Consider setting
> policies on important file systems such as copies=2.
>
> > Additionally I am wondering how the zfs mirror will handle a backend
> > disk failure on the Symmetrixs ?
> > with vxvm disk failures are transparent and nothing needs to be done
on
> > the host side.
>
> As long as the Symmetrix does not propagate the error to the host, then
> it should work the same. However, rather than just trust the storage,
> ZFS will verify the data with checksums. This is a good thing.
>
> > Will this be the same with zfs ? or will zpool replace commands need
to
> > be run ?
>
> As long as the data at the host is good, no ZFS commands are needed.
> However, there have been occasional reports of failed SAN hardware
> corrupting data. ZFS can detect this, and given enough redundancy, can
> correct the data.
> -- richard
>