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

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH] file-posix: add rough-block-status parameter
Date: Wed, 9 Jan 2019 18:09:29 +0100
User-agent: Mutt/1.10.1 (2018-07-13)

Am 09.01.2019 um 17:55 hat Paolo Bonzini geschrieben:
> 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.

preallocation=full doesn't do that. preallocation=falloc is more like
it, though that is just posix_fallocate(), not FALLOC_FL_ZERO_RANGE. But
when called on a new file, it might result in the same thing? Not sure.


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