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Re: [Qemu-devel] [Qemu-block] [RFC PATCH 0/8] qemu-img convert with copy

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [Qemu-block] [RFC PATCH 0/8] qemu-img convert with copy offloading
Date: Wed, 4 Apr 2018 14:23:32 +0100
User-agent: Mutt/1.9.2 (2017-12-15)

On Thu, Mar 29, 2018 at 07:09:06PM +0800, Fam Zheng wrote:
> [Posting a preview RFC for the general idea discussion and internal API 
> review.
> Libiscsi support is being worked on in the meantime.]
> This series introduces block layer API for copy offloading and makes use of it
> in qemu-img convert.
> For now we implemented the operation in local file protocol with
> copy_file_range(2).  Besides that it's possible to add similar to iscsi, nfs
> and potentially more.
> As far as its usage goes, in addition to qemu-img convert, we can emulate
> offloading in scsi-disk (EXTENDED COPY), and do similar to drive-mirror.
> The new bdrv_co_map_range can also be an alternative way to implement format
> drivers in the future, once we make block/io.c use it in preadv/pwritev paths.

I posted concerns about the bdrv_co_map_range() interface.  It would be
safer to only have a copy_range() interface without exposing how data is
mapped outside the driver where race conditions can occur and the format
driver no longer has full control over file layout.


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