Date: Tue, 22 Jan 2013 16:36:52 +0100 From: Borja Marcos <borjam@sarenet.es> To: Warren Block <wblock@wonkity.com> Cc: FreeBSD Filesystems <freebsd-fs@freebsd.org> Subject: Re: RFC: Suggesting ZFS "best practices" in FreeBSD Message-ID: <16B2C50C-DD36-4375-A002-F866A612D842@sarenet.es> In-Reply-To: <alpine.BSF.2.00.1301220759420.61512@wonkity.com> References: <314B600D-E8E6-4300-B60F-33D5FA5A39CF@sarenet.es> <alpine.BSF.2.00.1301220759420.61512@wonkity.com>
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On Jan 22, 2013, at 4:04 PM, Warren Block wrote: > I'm a proponent of using various types of labels, but my impression = after a recent experience was that ZFS metadata was enough to identify = the drives even if they were moved around. That is, ZFS bare metadata = on a drive with no other partitioning or labels. >=20 > Is that incorrect? I'm afraid it's inconvenient unless you enjoy reboots ;) This is a patologic and likely example I just demonstrated to a friend. We were testing a new server with 12 hard disks and a proper HBA. The disks are, unspririsingly, da0-da11. There is a da12 used (for now) = for the OS, so that there's no problem to create and destroy pools at = leisure. My friend had created a pool with two raidz vdevs nothing rocket = science. da0-5, da6-11. So, we were doing some tests and I've pulled one of the disks. Nothing = special, ZFS recovers nicely. Now it comes the fun part.=20 I reboot the machine with the missing disk. What happens now? I had pulled da4 I think. So, disks with an ID > 4 have been renamed to = N - 1. da5 became da4, da6 became da5, da7 became da6... and, = critically, da12 became da11. The reboot begun by failing to mount the root filesystem, but that one = is trivial. Just tell the kernel where it is now (da11) and it boots = happily. Now, we have a degraded pool with a missing disk (da4) and a da4 that = previously was da5. It works of course, but in degraded state. OK, we found a replacement disk, and we plugged it. It became, guess! = Yes, da12. Now: I cannot "online" da4, because it exists. I cannot online da12 = because it didn't belong to the pool. I cannot replace da4 with da12, = because it is there. Now that I think of it, in this case: 15896790606386444480 OFFLINE 0 0 0 was /dev/da4 Is it possible to say zpool replace 15896790606386444480 da12? I haven't = tried it. Anyway, seems to be a bit confusing. The logical, albeit cumbersome = approach is to reboot the machine with the new da4 in place, and after = rebooting, onlining or replacing.=20 Using names prevents this kind of confusion. Borja.
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