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Re: [Qemu-devel] [PATCH 4/7] block: initial VHDX driver support framewor
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH 4/7] block: initial VHDX driver support framework - supports open and probe |
Date: |
Thu, 7 Mar 2013 17:12:28 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Thu, Mar 07, 2013 at 10:23:54AM -0500, Jeff Cody wrote:
> On Thu, Mar 07, 2013 at 03:30:44PM +0100, Stefan Hajnoczi wrote:
> > On Wed, Mar 06, 2013 at 09:48:11AM -0500, Jeff Cody wrote:
> > > + ret = bdrv_pread(bs->file, s->bat_offset, s->bat, s->bat_rt.length);
> > > +
> > > + for (i = 0; i < s->bat_entries; i++) {
> > > + le64_to_cpus(&s->bat[i]);
> > > + }
> >
> > How does BAT size scale when the image size is increased? QCOW2 and QED
> > use caches for metadata that would be too large or wasteful to keep in
> > memory.
>
> The BAT size is dependent on the virtual disk size, and the block
> size. The block size is allowed to range from 1MB - 256MB. There is
> one BAT entry per block.
>
> In practice, the large block size keeps the BAT entry table reasonable
> (for a 127GB file, the block size created by Hyper-V is 32MB, so the
> table is pretty small - 32KB).
>
> However, I don't see anything in the spec that forces the block size
> to be larger, for a large virtual disk size. So for the max size of
> 64TB, and the smallest block size of 1MB, keeping the BAT in memory
> would indeed be excessive.
>
> I'll re-read the spec, and see if there is anything that ties the
> block size and virtual size together. If not, I'll have to add
> caching.
BTW the qcow2 cache code can be reused.
Stefan
[Qemu-devel] [PATCH 2/7] qemu: add castagnoli crc32c checksum algorithm, Jeff Cody, 2013/03/06
[Qemu-devel] [PATCH 3/7] block: vhdx header for the QEMU support of VHDX images, Jeff Cody, 2013/03/06
[Qemu-devel] [PATCH 4/7] block: initial VHDX driver support framework - supports open and probe, Jeff Cody, 2013/03/06
[Qemu-devel] [PATCH 5/7] block: add read-only support to VHDX image format., Jeff Cody, 2013/03/06
[Qemu-devel] [PATCH 6/7] block: add header update capability for VHDX images., Jeff Cody, 2013/03/06
[Qemu-devel] [PATCH 7/7] block: add write support for VHDX images, Jeff Cody, 2013/03/06