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List:       zfs-discuss
Subject:    Re: [zfs-discuss] repost - high read iops
From:       Bob Friesenhahn <bfriesen () simple ! dallas ! tx ! us>
Date:       2009-12-30 18:44:11
Message-ID: alpine.GSO.2.01.0912301233320.1586 () freddy ! simplesystems ! org
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On Wed, 30 Dec 2009, Richard Elling wrote:
> are written because you also assume that you can later read either side.  For
> ZFS, if only one side of the mirror is written, you know the bad side is bad
> because of the checksum. The checksum is owned by the parent, which is
> an important design decision that applies here, too.

In a mirror vdev, each mirror contains a complete copy of the vdev, 
and the last commited TXG can be used to know if the mirror is up to 
date with its peers, or if it needs a little bit of resilvering to 
catch up.  This differs from a raidz/raidz2 vdev, where most of the 
disks (N-1 or N-2) need to be available in order to have a complete 
copy of the vdev.

Bob
--
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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