Excerpts from Tomasz Chmielewski's message of 2010-11-18 09:57:34 -0500: > On 18.11.2010 15:23, Chris Mason wrote: > > 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? > > I guess you meant this during the second dd run: > > [27821.906513] btrfs-cache-8 D ffff88050c5fde98 0 8089 2 0x00000000 > [27821.906517] ffff88051c3a9b60 0000000000000046 ffff88051c3a9b00 ffff88051c3a9fd8 > [27821.906522] 00000000000139c0 00000000000139c0 ffff88051c3a9fd8 ffff88051c3a9fd8 > [27821.906526] 00000000000139c0 ffff88050c5fde98 ffff88050c5fdea0 ffff88050c5fdb00 > [27821.906530] Call Trace: > [27821.906534] [<ffffffff8159fc4e>] io_schedule+0x5e/0xa0 > [27821.906538] [<ffffffff81109f15>] sync_page+0x45/0x60 So, you're caching block groups. What you want to do is use Josef's new block group caching code. mount -o space_cache /dev/xxx Do the test and let the caching threads finish, then unmount and then your next run should be fast. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
