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

[Kevin Wolf <kwolf@redhat.com>]

Am 13.12.2016 um 11:16 hat Fam Zheng geschrieben:
> 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.

Is this a scenario that matters in practice, though? If you want
performance and you don't need COW functionality, just use raw images to
begin with.

Kevin