From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Mason Subject: Re: Interesting problem with write data. Date: Thu, 18 Nov 2010 09:23:47 -0500 Message-ID: <1290090160-sup-1717@think> References: <4CE51613.8060609@wpkg.org> Content-Type: text/plain; charset=UTF-8 Cc: linux-btrfs To: Tomasz Chmielewski Return-path: In-reply-to: <4CE51613.8060609@wpkg.org> List-ID: Excerpts from Tomasz Chmielewski's message of 2010-11-18 07:03:31 -0500: > > Recently, I made a btrfs to use. And I met slowness problem. Trying > > to diag it. I found this: > > 1. dd if=/dev/zero of=test count=1024 bs=1MB > > This is fast, at about 25MB/s, and reasonable iowait. > > 2. dd if=/dev/zero of=test count=1 bs=1GB > > This is pretty slow, at about 1.5MB/s, and 90%+ iowait, constantly. > > May I know why it works like this? Thanks. > > Reproducible here to some extent with 2.6.37-rc2. > > Interesting is, it only happens for me when I mount the filesystem, dd a smaller file, then a bigger one, in that order. > > Any subsequent dd / sync / rm usage doesn't seem to trigger it anymore (have to umount / mount again to trigger this). I'm going to guess this is the flushing threads, could you please run vmstat and see if there is a stream of reads? sysrq-w during the second dd would show it as well. -chris