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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Wed, 9 Jan 2019 17:55:41 +0100
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Subject: Re: [Qemu-devel] [PATCH] file-posix: add rough-block-status
 parameter
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org, armbru@redhat.com, eblake@redhat.com, mreitz@redhat.com, fam@euphon.net, stefanha@redhat.com, den@openvz.org

On 09/01/19 17:51, 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.

Sorry I mean at the protocol level, like FALLOC_FL_ZERO_RANGE.  It would
still require metadata updates on the filesystem level, unlike "real"
full preallocation, but no qcow2 metadata updates.
> 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.
> 
> Kevin
>