Re: Large qcow2 performance

[Andrew Martin]

----- Original Message -----
> From: "Alberto Garcia" <address@hidden>
> To: "qemu-discuss" <address@hidden>
> Sent: Wednesday, September 25, 2019 8:05:50 AM
> Subject: Re: Large qcow2 performance

> On Tue, Sep 24, 2019 at 02:45:10PM -0500, Andrew Martin wrote:
>> Hi Berto,
>> 
>> Thank you for the detailed explanation! It is also worth noting that
>> I am planning on hosting this qcow2 on a ZFS dataset (and ZFS does
>> some in-memory caching with the ARC as well); does that have an
>> impact on performance?
>> 
>> Moreover, would setting the cluster_size to match the ZFS recordsize
>> be advisable?
>> https://jrs-s.net/2018/03/13/zvol-vs-qcow2-with-kvm/
> 
> I don't have much experience with ZFS so I'm not the best to provide
> advice, but for the record I see that the ZFS recordsize is at most
> 128KB, whereas qcow2 supports clusters up to 2MB.
> 
> A 6TB qcow2 image can hold up to 384MB of L2 metadata if you use 128KB
> clusters. 384MB would be the largest possible size of the L2 cache,
> although in practice you'll probably be fine with much less, but how
> much less depends on your usage pattern.
> 
> Doubling the cluster size halves the size of the L2 metadata (and
> therefore the maximum needed cache size) but, as I said earlier, it
> makes allocation slower and increases the disk usage.
> 
> Berto

Hi Berto,

That makes sense, thanks for the additional details.

Andrew