Re: [Qemu-block] [PATCH v14 0/1] qcow2: cluster space preallocation

[Max Reitz]

On 26.05.19 17:01, Alberto Garcia wrote:
> On Fri 24 May 2019 03:56:21 PM CEST, Max Reitz <address@hidden> wrote:
>>>     +-----------+-------+------+-------+------+------+
>>>     |   file    |    before    |     after    | gain |
>>>     +-----------+-------+------+-------+------+------+
>>>     |    ssd    |      61.153  |      36.313  |  41% |
>>>     |    hdd    |     112.676  |     122.056  |  -8% |
>>>     +-----------+--------------+--------------+------+
>>
>> I’ve done a few more tests, and I’ve seen more slowdown on an HDD.
>> (Like 30 % when doing 64 kB requests that are not aligned to
>> clusters.)  On the other hand, the SSD gain is generally in the same
>> ballpark (38 % when issuing the same kind of requests.)
>   [...]
>> [1] Hm.  We can probably investigate whether the file is stored on a
>> rotational medium or not.  Is there a fundamental reason why this
>> patch seems to degrade performance on an HDD but improves it on an
>> SSD?  If so, we can probably make a choice based on that.
> 
> This is when writing to an unallocated cluster with no existing data on
> the backing image, right? Then it's probably because you need 2
> operations (write zeros + write data) instead of just one.

Hm, yes.  I didn’t test writing tail and head separately, which should
be even worse.

Max

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