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From: Daniel Kalchev <daniel@digsys.bg>
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Date: Tue, 20 Dec 2011 00:31:53 +0200
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To: Dan Nelson <dnelson@allantgroup.com>
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Cc: FreeBSD Current <freebsd-current@freebsd.org>
Subject: Re: Uneven load on drives in ZFS RAIDZ1
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On Dec 19, 2011, at 11:53 PM, Dan Nelson wrote:

>=20
> Since it looks like the algorithm ends up creating two half-cold =
parity
> disks instead of one cold disk, I bet a 3-disk RAIDZ would exhibit =
even
> worse balancing, and a 5-disk set would be more even.

There were some experiments a year or two ago with different number of =
disks in raidz and the results suggested that certain number of disks =
had better performance, contrary to theory that writes should be evenly =
distributed. Worse, this is in the official theory of how raidz =
operates=85

Perhaps the code can be fixed to spread the writes to all devices in =
raidz, but compatibility issues need to be considered.

Perhaps DDT is stored in the 'worst case' write size, because it clearly =
exhibits such poor distribution.

Daniel=