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Re: [Qemu-devel] [PATCH] file-posix: add rough-block-status parameter

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] file-posix: add rough-block-status parameter
Date: Wed, 9 Jan 2019 11:00:31 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

On 1/9/19 10:51 AM, Kevin Wolf wrote:
> Am 09.01.2019 um 17:42 hat Paolo Bonzini geschrieben:
>> On 09/01/19 12:23, Kevin Wolf wrote:
>>> Also note that this is only metadata preallocation; full preallocation
>>> will still return allocated for the protocol layer and so it will always
>>> be slow.
>> Full preallocation these days can create images with preallocated but
>> known-zero blocks, I think?
> That would defeat one of the main purposes of preallocation because it
> would still require COW and metadata updates on the first write.
> If there is demand, we could add something like preallocation=data where
> data clusters are preallocated but COW/metadata updates still happen at
> runtime, but so far nobody has asked for it. Not sure when you would use
> it either.

Except there HAS been talk about it before - such a mode makes it so
that you can guarantee a non-fragmented image, or even guarantee that
the entire guest-visible portion is contiguous and all qcow2 metadata is
at the front or back of the overall qcow2 file, making it very easy to
convert between qcow2 -> raw (strip the qcow2 metadata) or go in reverse
(wrap a raw into a qcow2) without having to shuffle the guest-visible
portions of the file.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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