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

[Alberto Garcia]

On Fri 21 Aug 2020 01:42:52 PM CEST, Alberto Garcia wrote:
> On Fri 21 Aug 2020 01:05:06 PM CEST, Brian Foster <bfoster@redhat.com> wrote:
>>> > 1) off: for every write request QEMU initializes the cluster (64KB)
>>> >         with fallocate(ZERO_RANGE) and then writes the 4KB of data.
>>> > 
>>> > 2) off w/o ZERO_RANGE: QEMU writes the 4KB of data and fills the rest
>>> >         of the cluster with zeroes.
>>> > 
>>> > 3) metadata: all clusters were allocated when the image was created
>>> >         but they are sparse, QEMU only writes the 4KB of data.
>>> > 
>>> > 4) falloc: all clusters were allocated with fallocate() when the image
>>> >         was created, QEMU only writes 4KB of data.
>>> > 
>>> > 5) full: all clusters were allocated by writing zeroes to all of them
>>> >         when the image was created, QEMU only writes 4KB of data.
>>> > 
>>> > As I said in a previous message I'm not familiar with xfs, but the
>>> > parts that I don't understand are
>>> > 
>>> >    - Why is (4) slower than (1)?
>>> 
>>> Because fallocate() is a full IO serialisation barrier at the
>>> filesystem level. If you do:
>>> 
>>> fallocate(whole file)
>>> <IO>
>>> <IO>
>>> <IO>
>>> .....
>>> 
>>> The IO can run concurrent and does not serialise against anything in
>>> the filesysetm except unwritten extent conversions at IO completion
>>> (see answer to next question!)
>>> 
>>> However, if you just use (4) you get:
>>> 
>>> falloc(64k)
>>>   <wait for inflight IO to complete>
>>>   <allocates 64k as unwritten>
>>> <4k io>
>>>   ....
>>> falloc(64k)
>>>   <wait for inflight IO to complete>
>>>   ....
>>>   <4k IO completes, converts 4k to written>
>>>   <allocates 64k as unwritten>
>>> <4k io>
>>> falloc(64k)
>>>   <wait for inflight IO to complete>
>>>   ....
>>>   <4k IO completes, converts 4k to written>
>>>   <allocates 64k as unwritten>
>>> <4k io>
>>>   ....
>>> 
>>
>> Option 4 is described above as initial file preallocation whereas
>> option 1 is per 64k cluster prealloc. Prealloc mode mixup aside, Berto
>> is reporting that the initial file preallocation mode is slower than
>> the per cluster prealloc mode. Berto, am I following that right?

After looking more closely at the data I can see that there is a peak of
~30K IOPS during the first 5 or 6 seconds and then it suddenly drops to
~7K for the rest of the test.

I was running fio with --ramp_time=5 which ignores the first 5 seconds
of data in order to let performance settle, but if I remove that I can
see the effect more clearly. I can observe it with raw files (in 'off'
and 'prealloc' modes) and qcow2 files in 'prealloc' mode. With qcow2 and
preallocation=off the performance is stable during the whole test.

Berto