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

[Alberto Garcia]

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.

Berto