Date: Tue, 20 Dec 2011 00:31:53 +0200 From: Daniel Kalchev <daniel@digsys.bg> To: Dan Nelson <dnelson@allantgroup.com> Cc: FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: Uneven load on drives in ZFS RAIDZ1 Message-ID: <F5284C29-DD7C-43A1-9CA3-71040ACF9536@digsys.bg> In-Reply-To: <20111219215317.GL53453@dan.emsphone.com> References: <4EEF488E.1030904@freebsd.org> <20111219162220.GK53453@dan.emsphone.com> <4EEFA05E.7090507@freebsd.org> <20111219215317.GL53453@dan.emsphone.com>
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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=
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