Re: [PATCH 0/2] qcow2: Force preallocation with data-file-raw

[Kevin Wolf]

Am 23.06.2020 um 09:28 hat Max Reitz geschrieben:
> On 22.06.20 19:44, Alberto Garcia wrote:
> > On Mon 22 Jun 2020 11:47:32 AM CEST, Max Reitz wrote:
> >>> I don't know the internals of qcow2 data_file, but are we really using
> >>> qcow2 metadata when accessing the data file?
> >>
> >> Yes.
> >>
> >>> This may have unwanted performance consequences.
> >>
> >> I don’t think so, because in practice normal lookups of L1/L2 mappings
> >> generally don’t cost that much performance.
> > 
> > ...if the L2 cache size is large enough. Otherwise you need one extra
> > read operation to retrieve the L2 metadata.
> > 
> > Possible performance problems when you have preallocation:
> > 
> >    - If a block hasn't been written yet (it's all zeroes) then you still
> >      need to read the L2 entry and read the data block. If there is not
> >      L2 table then you can simply return zeroes without going to disk at
> >      all. This of course assumes that the contents of the unwritten data
> >      block are zeroes.
> > 
> >    - QEMU still needs to read from disk (and cache in memory) the L2
> >      metadata, when it already knows in advance the contents of the L2
> >      entry (guest_offset == host_offset).
> 
> We could well optimize this regardless of preallocation.  With
> data-file-raw, qemu doesn’t have to look at the L2 metadata at all.
> 
> So the problem isn’t preallocation at all, it’s the fact that we don’t
> have such an optimization.  But note that to implement such an
> optimization, we really do need preallocation: Because it would mean
> that we wouldn’t touch the L1/L2 tables for data-file-raw images during
> runtime, which would effectively make those images empty to today’s qemu
> versions.

It depends. For reads, bypassing the L1/L2 tables is completely fine
with data-file-raw. It may miss opportunities to optimise reading
unallocated/zeroed clusters, but if the data file is actually sparse, it
shouldn't make a big difference. Maybe we should just do this.

For (potentially allocating) writes, you're right that we need to be
more careful. If we want to completely bypass L1/L2 tables,
preallocation is not enough, but we have to make sure that we never
discard any clusters.

Whatever we do for writes will be a non-trivial change. I wonder if it's
really worth doing this for optimisation when nobody uses the feature
yet anyway.

> (OTOH, preallocation would then be pretty much superfluous for all newer
> versions of qemu.  To address that, we could then add an incompatible
> version of data-file-raw.  But I think we should only think about that
> once we get to that point.)

Well, if we create an incompatible version, we can have one that doesn't
even store L1/L2 tables.

Kevin

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