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Date: Mon, 30 Nov 2009 15:08:35 -0600
From: "James R. Van Artsdalen" <james-freebsd-fs2@jrv.org>
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To: Zaphod Beeblebrox <zbeeble@gmail.com>
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Cc: freebsd-fs <freebsd-fs@freebsd.org>
Subject: Re: ZFS guidelines - preparing for future storage expansion
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Zaphod Beeblebrox wrote:
> I moved from 5x 750G to 5x 1.5T disks this way earlier this year. 
> [...] And keep in mind that while you're upgrading, you're vulnerable
> to data loss (no more replicas).

This is one of the (many) reasons I prefer mirrors rather than parity
(RAID-5).  You can "attach" the new drive, wait for the resilver to
complete, then detach the old drive - never having fewer than two drives
in the mirror.  And of course you can gain space in the pool as each
mirror is upgraded whereas a parity group (RAIDZ) usually involves more
drives.

Note that the zpool(1) man page says of the "Replace" command:
"Replaces  old_device with new_device. This is equivalent to attaching
new_device, waiting for it  to  resilver,  and  then  detaching
old_device".  This is not quite true: the reads for the resilver come
from all available devices if you do attach/detach, but do not come from
old_device if you do "replace".  This is for MIRRORs; I'm not sure how
RAIDZ behaves.