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Re: [Qemu-block] [Qemu-devel] [PATCH V20 0/7] add-cow file format

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH V20 0/7] add-cow file format
Date: Tue, 23 Feb 2016 06:33:22 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

Reviving an old thread for discussion purposes:

On 08/25/2013 10:06 AM, Dong Xu Wang wrote:
> It will introduce a new file format: add-cow.
> The add-cow file format makes it possible to perform copy-on-write on top of
> a raw disk image.  When we know that no backing file clusters remain visible
> (e.g. we have streamed the entire image and copied all data from the backing
> file), then it is possible to discard the add-cow file and use the raw image
> file directly.
> This feature adds the copy-on-write feature to raw files (which cannot support
> it natively) while allowing us to get full performance again later when we no
> longer need copy-on-write.

Lately there has been discussion about a new external bitmap file
format. I suspect everything in that conversation will make it possible
to do anything that was planned by this (now-abandoned) series.  But
knowing all the use cases may help make sure we consider all the design
aspects for what is needed if we track bitmaps of raw files in their own
file format.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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