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Re: [Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support
Date: Wed, 13 Oct 2010 08:24:13 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100915 Lightning/1.0b1 Thunderbird/3.0.8

On 10/13/2010 08:07 AM, Kevin Wolf wrote:
Am 13.10.2010 14:13, schrieb Stefan Hajnoczi:
We can avoid it when a backing image is not used.  Your idea to check
for zeroes in the backing image is neat too, it may well reduce the
common case even for backing images.
The additional requirement is that we're extending the file and not
reusing an old cluster. (And bdrv_has_zero_init() == true, but QED
doesn't work on host_devices anyway)

Yes, that's a good point.

BTW, I think we've decided that making it work on host_devices is not that bad.

We can add an additional feature called QED_F_PHYSICAL_SIZE.

This feature will add another field to the header that contains an offset immediately following the last cluster allocation.

During a metadata scan, we can accurately recreate this field so we only need to update this field whenever we clear the header dirty bit (which means during an fsync()).

That means we can maintain the physical size without introducing additional fsync()s in the allocation path. Since we're already writing out the header anyway, the write operation is basically free too.


Anthony Liguori


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