Re: [Qemu-devel] [PATCHv2] block: introduce BDRV_O_SEQUENTIAL

[Peter Lieven <pl@kamp.de>]

On 21.03.2014 13:06, Paolo Bonzini wrote:
> Il 21/03/2014 12:49, Peter Lieven ha scritto:
>> A 10G logical volume was created and filled with random data.
>> Then the logical volume was exported via qemu-img convert to an iscsi target.
>> Before the export was started all caches of the linux kernel where dropped.
>>
>> Old behavior:
>>  - The convert process took 3m45s and the buffer cache grew up to 9.67 GB close
>>    to the end of the conversion. After qemu-img terminated all the buffers were
>>    freed by the kernel.
>>
>> New behavior with the -N switch:
>>  - The convert process took 3m43s and the buffer cache grew up to 15.48 MB close
>>    to the end with some small peaks up to 30 MB durine the conversion.
>>
>> Signed-off-by: Peter Lieven <pl@kamp.de>
>> ---
>> v1->v2: - added test example to commit msg
>>         - added -N knob to qemu-img
>
> I'm sorry, I cannot find the original discussion.  Why is the new knob needed?

The thread was named "qemu-img convert cache mode for source".

I think the 2 points (mainly by Marcus) were that you would not expect qemu-img
to mangle with the page cache by default as you would not expect it from cp or dd.
And secondly, if a running vServer and the image that is converted share pages it
can ruin the vServers cache.

Peter