Re: [PATCH 0/1] qcow2: Skip copy-on-write when allocating a zero cluster

[Alberto Garcia]

On Mon 17 Aug 2020 05:53:07 PM CEST, Kevin Wolf wrote:
>> > Or are you saying that ZERO_RANGE + pwrite on a sparse file (=
>> > cluster allocation) is faster for you than just the pwrite alone (=
>> > writing to already allocated cluster)?
>> 
>> Yes, 20% faster in my tests (4KB random writes), but in the latter
>> case the cluster is already allocated only at the qcow2 level, not on
>> the filesystem. preallocation=falloc is faster than
>> preallocation=metadata (preallocation=off sits in the middle).
>
> Hm, this feels wrong. Doing more operations should never be faster
> than doing less operations.
>
> Maybe the difference is in allocating 64k at once instead of doing a
> separate allocation for every 4k block? But with the extent size hint
> patches to file-posix, we should allocate 1 MB at once by default now
> (if your test image was newly created). Can you check whether this is
> in effect for your image file?

I checked with xfs on my computer. I'm not very familiar with that
filesystem so I was using the default options and I didn't tune
anything.

What I got with my tests (using fio):

- Using extent_size_hint didn't make any difference in my test case (I
  do see a clear difference however with the test case described in
  commit ffa244c84a).

- preallocation=off is still faster than preallocation=metadata. If I
  disable handle_alloc_space() (so there is no ZERO_RANGE used) then it
  is much slower.

- With preallocation=falloc I get the same results as with
  preallocation=metadata.

- preallocation=full is the fastest by far.

Berto