Date: Tue, 8 Mar 2011 03:48:10 -0800 From: Jeremy Chadwick <freebsd@jdc.parodius.com> To: "Patrick M. Hausen" <hausen@punkt.de> Cc: freebsd-stable <freebsd-stable@freebsd.org> Subject: Re: ZFS performance as the FS fills up? Message-ID: <20110308114810.GA37554@icarus.home.lan> In-Reply-To: <BD2C8439-AC1E-49CB-ABE2-9F7F0B292933@punkt.de> References: <BD2C8439-AC1E-49CB-ABE2-9F7F0B292933@punkt.de>
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On Tue, Mar 08, 2011 at 12:26:49PM +0100, Patrick M. Hausen wrote: > we use a big JBOD and ZFS with raidz2 as the target > for our nightly Amanda backups. > > I already suspected that the fact that the FS was > 90% full might > be the cause of our backup performance continously decreasing. > > I just added another vdev - 6 disks of 750 GB each, raidz2 and the > FS usage is back to 71% currently. This was while backups were > running and write performance instantly skyrocketed compared to > the values before. > > So, is it possible to name a reasonable amount of free space to > keep on a raidz2 volume? On last year's EuroBSDCon I got > the impression that with recent (RELENG_8) ZFS merges > I could get away with using around 90%. I'm in no way attempting to dissuade you from your efforts to figure out a good number for utilisation, but when I hear of disks -- no matter how many -- being 90% full, I immediately conclude performance is going to suck simply because the outer "tracks" on a disk contains more sectors than the inner "tracks". This is the reason for performance degradation as the seek offset increases, resulting in graphs like this: http://img171.imageshack.us/img171/4776/1tb2.png Given this info, it shouldn't come as too much of a surprise that adding another vdev (effectively adding more disks to the pool) greatly helps in relieving this issue. -- | Jeremy Chadwick jdc@parodius.com | | Parodius Networking http://www.parodius.com/ | | UNIX Systems Administrator Mountain View, CA, USA | | Making life hard for others since 1977. PGP 4BD6C0CB |
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