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[Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_syn

From: Anthony Liguori
Subject: [Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"
Date: Wed, 25 Aug 2010 08:42:19 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100713 Lightning/1.0b1 Thunderbird/3.0.6

On 08/25/2010 08:23 AM, Avi Kivity wrote:
 On 08/25/2010 03:46 PM, Anthony Liguori wrote:

If we had another disk format that only supported growth and metadata for a backing file, can you think of another failure scenario?

btw, only supporting growth is a step backwards. Currently file-backed disks keep growing even the guest-used storage doesn't grow, since once we allocate something we never release it. But eventually guests will start using TRIM or DISCARD or however it's called, and then we can expose it and reclaim unused blocks.

You can do this in one of two ways. You can do online compaction or you can maintain a free list. Online compaction has an advantage because it does not require any operations in the fast path whereas a free list would require ordered metadata updates (must remove something from the first list before updating the l2 table) which implies a sync.

At a high level, I don't think online compaction requires any specific support from an image format.


Anthony Liguori

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