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    SubjectRe: BTRFS: Unbelievably slow with kvm/qemu
    On 07/12/2010 08:24 AM, Giangiacomo Mariotti wrote:
    > Hi, is it a known problem how much slow is Btrfs with kvm/qemu(meaning
    > that the image kvm/qemu uses as the hd is on a partition formatted
    > with Btrfs, not that the fs used by the hd inside the kvm environment
    > is Btrfs, in fact inside kvm the / partition is formatted with ext3)?
    > I haven't written down the exact numbers, because I forgot, but while
    > I was trying to make it work, after I noticed how much longer than
    > usual it was taking to just install the system, I took a look at iotop
    > and it was reporting a write speed of the kvm process of approximately
    > 3M/s, while the Btrfs kernel thread had an approximately write speed
    > of 7K/s! Just formatting the partitions during the debian installation
    > took minutes. When the actual installation of the distro started I had
    > to stop it, because it was taking hours! The iotop results made me
    > think that the problem could be Btrfs, but, to be sure that it wasn't
    > instead a kvm/qemu problem, I cut/pasted the same virtual hd on an
    > ext3 fs and started kvm with the same parameters as before. The
    > installation of debian inside kvm this time went smoothly and fast,
    > like normally it does. I've been using Btrfs for some time now and
    > while it has never been a speed champion(and I guess it's not supposed
    > to be one and I don't even really care that much about it), I've never
    > had any noticeable performance problem before and it has always been
    > quite stable. In this test case though, it seems to be doing very bad.

    Btrfs is very slow on sync writes:

    $ fio --name=x --directory=/images --rw=randwrite --runtime=300
    --size=1G --filesize=1G --bs=4k --ioengine=psync --sync=1 --unlink=1
    x: (g=0): rw=randwrite, bs=4K-4K/4K-4K, ioengine=psync, iodepth=1
    Starting 1 process
    x: Laying out IO file(s) (1 file(s) / 1024MB)
    Jobs: 1 (f=1): [w] [1.3% done] [0K/0K /s] [0/0 iops] [eta 06h:18m:45s]
    x: (groupid=0, jobs=1): err= 0: pid=2086
    write: io=13,752KB, bw=46,927B/s, iops=11, runt=300078msec
    clat (msec): min=33, max=1,711, avg=87.26, stdev=60.00
    bw (KB/s) : min= 5, max= 105, per=103.79%, avg=46.70, stdev=15.86
    cpu : usr=0.03%, sys=19.55%, ctx=47197, majf=0, minf=94
    IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%,
    submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
    complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
    issued r/w: total=0/3438, short=0/0

    lat (msec): 50=3.40%, 100=75.63%, 250=19.14%, 500=1.40%, 750=0.35%
    lat (msec): 1000=0.06%, 2000=0.03%

    Run status group 0 (all jobs):
    WRITE: io=13,752KB, aggrb=45KB/s, minb=46KB/s, maxb=46KB/s,
    mint=300078msec, maxt=300078msec

    45KB/s, while 4-5MB/s traffic was actually going to the disk. For every
    4KB that the the application writes, 400KB+ of metadata is written.

    (It's actually worse, since it starts faster than the average and ends
    up slower than the average).

    For kvm, you can try cache=writeback or cache=unsafe and get better
    performance (though still slower than ext*).

    I have a truly marvellous patch that fixes the bug which this
    signature is too narrow to contain.

     \ /
      Last update: 2010-07-13 06:33    [W:0.023 / U:0.588 seconds]
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