Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk

[Fam Zheng <famz@redhat.com>]

On Tue, 12/13 09:02, Max Reitz wrote:
> > Yes but the use case is that the qcow2 image is stored in a slow disk,
> > so things will be faster if we avoid having to read it too often.
> > 
> > But the data is there and it needs to be read, so we have three options:
> > 
> >   1) Read it everytime we need it. It slows things down.
> >   2) Keep (part of) it in memory. It can use a lot of memory.
> >   3) Keep it in a faster disk.
> > 
> > We're talking about 3) here, and this it not about creating new
> > structures, but about changing the storage backend of the existing L2
> > cache (disk rather than RAM).
> 
> I'm arguing that we already have an on-disk L2 structure and that is called
> simply the L1-L2 structure in the qcow2 file. The cache only makes sense
> because it is in RAM.

I am looking at this problem from a different angle:

Assume a qcow2 image created by a known version of QEMU with the option
preallocation=falloc, it is theoretically possible to do a hacky optimization in
qcow2 I/O path that, instead of looking up in L1/L2, we can calculate file
offset for any given virtual address, by adding the file offset of LBA 0.

If we make this optimization a header extention and add it to the spec, it is no
longer a hack, and it should offer nearly the same performance as raw in all
cases - if we say raw is doing "identical mapping" from LBA to file offset, this
new operation mode of qcow2 file is basically doing a "shift by constant" mapping.

Fam