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Re: Large qcow2 performance


From: Alberto Garcia
Subject: Re: Large qcow2 performance
Date: Wed, 25 Sep 2019 15:05:50 +0200
User-agent: Mutt/1.5.23 (2014-03-12)

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



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