Date: Thu, 16 Apr 2009 21:32:45 -0400 From: Ben Kelly <ben@wanderview.com> To: Peter Schuller <peter.schuller@infidyne.com> Cc: freebsd-current@freebsd.org, Artem Belevich <fbsdlist@src.cx> Subject: Re: [patch] zfs livelock and thread priorities Message-ID: <B9234D72-B5A9-4A35-940F-F70AA02BBFFB@wanderview.com> In-Reply-To: <20090416132302.GA86096@hyperion.scode.org> References: <DC9F2088-A0AF-467D-8574-F24A045ABD81@wanderview.com> <49C2CFF6.8070608@egr.msu.edu> <BDABA909-C2AE-4A55-869B-CA01BE778A82@wanderview.com> <ed91d4a80904131636u18c90474w7cdaa57bc7000e02@mail.gmail.com> <08D7DC2A-68BE-47B6-8D5D-5DE6B48F87E5@wanderview.com> <AC3C4C3F-40C6-4AF9-BAF3-2C4D1E444839@wanderview.com> <ed91d4a80904142135n429dea52o672abf51116fa707@mail.gmail.com> <20090416132302.GA86096@hyperion.scode.org>
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On Apr 16, 2009, at 9:23 AM, Peter Schuller wrote: >> BTW, now that you're tinkering with ZFS threads and priorities, >> whould >> you by any chance have any idea why zfs scrub is so painfully slow on >> -current? >> When I start scrub on my -stable box, it pretty much runs full speed >> -- I can see disks under load all the time. >> However on -current scrub seems to run in small bursts. Disks get >> busy >> for a second or so and then things get quiet for about five seconds >> or >> so and this pattern repeats over and over. > > This is intentional. The newer ZFS code has, if I remember correctly, > something like "spend at most 1/5 of the time doing scrub for each > underlying vdev". I could be wrong on the details and I don't have > source refs off-hand, by I looked into this when I wanted to see if I > could tweak this (while I definitely like it rate limited, I would > have liked to up the threshold a bit). My conclusion at the time was > that there was no way to tweak it other than recompiling the kernel. I find it odd that they would pause for up to 5 seconds, though. I would expect the throttling to occur over a much smaller time span. Do you see similarly large pauses in disk I/O during a scrub? - Ben
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