[Qemu-devel] Re: slow ext4 O_SYNC writes (why qemu qcow2 is so slow on ext4 vs ext3)

[Jan Kara]

  Hi,

On Fri 02-07-10 16:46:28, Michael Tokarev wrote:
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> I noticed that qcow2 images, esp. fresh ones (so that they
> receive lots of metadata updates) are very slow on my
> machine.  And on IRC (#kvm), Sheldon Hearn found that on
> ext3, it is fast again.
> 
> So I tested different combinations for a bit, and observed
> the following:
> 
> for fresh qcow2 file, with default qemu cache settings,
> copying kernel source is about 10 times slower on ext4
> than on ext3.  Second copy (rewrite) is significantly
> faster in both cases (expectable), but still ~20% slower
> on ext4 than on ext3.
> 
> Normal cache mode in qemu is writethrough, which translates
> to O_SYNC file open mode.
> 
> With cache=none, which translates to O_DIRECT, metadata-
> intensive writes (fresh qcow) are about as slow as on
> ext4 with O_SYNC, and rewrite is expectedly faster, but
> now there's _no_ difference in speed between ext3 and ext4.
> 
> I did a series of straces of the writer processes, -- time
> spent in pwrite() syscalls is significantly larger for
> ext4 with O_SYNC than with ext3 with O_SYNC, the diff is
> about 50 times.
> 
> Also, with slower I/O in case of ext4, qemu-kvm starts more
> I/O threads, which, as it seems, slows whole thing down even
> further - I changed max_threads from default 64 to 16, and
> the speed improved slightly.  Here, the diff. is again quite
> significant: on ext3 qemu spawns only 8 threads, while on
> ext4 all 64 I/O threads are spawned almost immediately.
> 
> So I've two questions:
> 
>  1.  Why ext4 O_SYNC is too slow compared with ext3 O_SYNC?
>    This is observed on 2.6.32 and 2.6.34 kernels, barriers
>    or data={writeback|ordered} had no difference.  I tested
>    whole thing on a partition on a single drive, sheldonh
>    used ext[34]fs on top of lvm on a raid1 volume.
  Do I get it right, that you have ext3/4 which carries fs images used by
KVM? What you describe is strange. Up to this moment it sounded to me like
a difference in barrier settings on the host but you seem to have tried
that. Just stabbing in the dark - could you try nodelalloc mount option
of ext4?

>  2.  The number of threads spawned for I/O... this is a good
>    question, how to find an adequate cap.  Different hw has
>    different capabilities, and we may have more users doing
>    I/O at the same time...
  Maybe you could measure your total throughput over some period,
try increasing number of threads in the next period and if it
helps significantly, use larger number, otherwise go back to a
smaller number?

                                                        Honza
-- 
Jan Kara <address@hidden>
SUSE Labs, CR