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Re: [Qemu-block] [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2
From: |
Fam Zheng |
Subject: |
Re: [Qemu-block] [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk |
Date: |
Tue, 13 Dec 2016 18:16:35 +0800 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
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
- [Qemu-block] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Alberto Garcia, 2016/12/09
- [Qemu-block] [PATCH RFC 1/1] qcow2: Allow storing the qcow2 L2 cache in disk, Alberto Garcia, 2016/12/09
- Re: [Qemu-block] [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, no-reply, 2016/12/09
- Re: [Qemu-block] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Kevin Wolf, 2016/12/09
- Re: [Qemu-block] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Max Reitz, 2016/12/09
- Re: [Qemu-block] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Alberto Garcia, 2016/12/13
- Re: [Qemu-block] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Max Reitz, 2016/12/13
- Re: [Qemu-block] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Alberto Garcia, 2016/12/13
Re: [Qemu-block] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Stefan Hajnoczi, 2016/12/12