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: > > 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… 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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