Re: qcow2 perfomance: read-only IO on the guest generates high write IO on the host

[Christopher Pereira <kripper@imatronix.cl>]

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On 24-08-2021 11:37, Kevin Wolf wrote:
> [ Cc: qemu-block ]
>
> Am 11.08.2021 um 13:36 hat Christopher Pereira geschrieben:
>> Hi,
>>
>> I'm reading a directory with 5.000.000 files (2,4 GB) inside a guest using
>> "find | grep -c".
>>
>> On the host I saw high write IO (40 MB/s !) during over 1 hour using
>> virt-top.
>>
>> I later repeated the read-only operation inside the guest and no additional
>> data was written on the host. The operation took only some seconds.
>>
>> I believe QEMU was creating some kind of cache or metadata map the first
>> time I accessed the inodes.
> No, at least in theory, QEMU shouldn't allocate anything when you're
> just reading.
Hmm...interesting.
> Are you sure that this isn't activity coming from your guest OS?

Yes. iotop was showing only read IOs on the guest, and on the host iotop 
and virt-top where showing strong write IOs for hours.
Stopping the "find" command on the guest also stopped the write IOs on 
the host.

>> But I wonder why the cache or metadata map wasn't available the first time
>> and why QEMU had to recreate it?
>>
>> The VM has "compressed base <- snap 1" and base was converted without
>> prealloc.
>>
>> Is it because we created the base using convert without metadata prealloc
>> and so the metadata map got lost?
>>
>> I will do some experiments soon using convert + metadata prealloc and
>> probably find out myself, but I will happy to read your comments and gain
>> some additional insights.
>> If it the problem persists, I would try again without compression.
> What were the results of your experiments? Is the behaviour related to
> any of these options?

I will do the experiments and report back.

It's also strange that the second time I repeat the "find" command, I 
see no more write IOs and it takes only seconds instead of hours.

I was assuming QEMU was creating some kind of map or cache on the 
snapshot for the content present in the base, but now I got more curious.


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