Date: Mon, 21 Jul 2008 21:18:31 -0700 From: Steven Schlansker <stevenschlansker@berkeley.edu> To: freebsd-questions@freebsd.org Subject: Using ccd with zfs Message-ID: <DD98FA1F-04E2-40C3-BF97-7F80ACBB1006@berkeley.edu>
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Hello -questions, I have a FreeBSD ZFS storage system working wonderfully with 7.0. It's set up as three 3-disk RAIDZs -triplets of 500, 400, and 300GB drives. I recently purchased three 750GB drives and would like to convert to using a RAIDZ2. As ZFS has no restriping capabilities yet, I will have to nuke the zpool from orbit and make a new one. I would like to verify my methodology against your experience to see if what I wish to do is reasonable: I plan to first take 2 of the 750GB drives and make an unreplicated 1.5TB zpool as a temporary storage. Since ZFS doesn't seem to have the ability to create zpools in degraded mode (with missing drives) I plan to use iSCSI to create two additional drives (backed by /dev/ zero) to fake having two extra drives, relying on ZFS's RAIDZ2 protection to keep everything running despite the fact that two of the drives are horribly broken ;) To make these 500, 400, and 300GB drives useful, I would like to stitch them together using ccd. I would use it as 500+300 = 800GB and 400+400=800GB That way, in the end I would have 750 x 3 500 + 300 x3 400 + 400 x 1 400 + 200 + 200 x 1 as the members in my RAIDZ2 group. I understand that this is slightly less reliable than having "real" drives for all the members, but I am not interested in purchasing 5 more 750GB drives. I'll replace the drives as they fail. I am wondering if there are any logistical problems. The three parts I am worried about are: 1) Are there any problems with using an iSCSI /dev/zero drive to fake drives for creation of a new zpool, with the intent to replace them later with proper drives? 2) Are there any problems with using CCD under zpool? Should I stripe or concatenate? Will the startup scripts (either by design or less likely intelligently) decide to start CCD before zfs? The zpool should start without me interfering, correct? 3) I hear a lot about how you should use whole disks so ZFS can enable write caching for improved performance. Do I need to do anything special to let the system know that it's OK to enable the write cache? And persist across reboots? Any other potential pitfalls? Also, I'd like to confirm that there's no way to do this pure ZFS-like - I read the documentation but it doesn't seem to have support for nesting vdevs (which would let me do this without ccd) Thanks for any information that you might be able to provide, Steven Schlansker
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