Date: Wed, 25 Jul 2007 19:57:36 +0100 From: Doug Rabson <dfr@rabson.org> To: Bakul Shah <bakul@bitblocks.com> Cc: freebsd-fs@freebsd.org, Pawel Jakub Dawidek <pjd@freebsd.org>, Mark Powell <M.S.Powell@salford.ac.uk> Subject: Re: ZfS & GEOM with many odd drive sizes Message-ID: <1185389856.3698.11.camel@herring.rabson.org> In-Reply-To: <20070725174715.9F47E5B3B@mail.bitblocks.com> References: <20070725174715.9F47E5B3B@mail.bitblocks.com>
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On Wed, 2007-07-25 at 10:47 -0700, Bakul Shah wrote: > > If you do that, ZFS can use its checksums to continually > > monitor the two sides of your mirrors for consistency and will be able > > to notice as early as possible when one of the drives goes flakey. > > Does it really do this? As I understood it, only one of the > disks in a mirror will be read for a given block. If the > checksum fails, the same block from the other disk is read > and checksummed. If all the disks in a mirror are read for > every block, ZFS read performance would get somewhat worse > instead of linear scaling up with more disks in a mirror. In > order to monitor data on both disks one would need to > periodically run "zpool scrub", no? But that is not > *continuous* monitoring of the two sides. This is of course correct. I should have said "continuously checks the data which you are actually looking at on a regular basis". The consistency check is via the block checksum (not comparing the date from the two sides of the mirror).help
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