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Re: [Qemu-devel] [PATCH RFC for-3.0-rc3 0/3] qemu-img: Disable copy offl

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH RFC for-3.0-rc3 0/3] qemu-img: Disable copy offloading by default
Date: Fri, 27 Jul 2018 12:29:38 +0200
User-agent: Mutt/1.9.1 (2017-09-22)

Am 27.07.2018 um 05:33 hat Fam Zheng geschrieben:
> Kevin pointed out that both glibc and kernel provides a slow fallback of
> copy_file_range which hurts thin provisioning. This is particularly true for
> thin LVs, because host_device driver cannot get allocation info from the
> volume, and copy_file_range is called on every sectors, making the dst fully
> allocated.
> NFS mount points also doesn't support SEEK_DATA well, so the allocation
> information is unknown to QEMU.
> That leaves only iscsi:// which seems to do what we want so far, but it is a
> smaller use case.
> Add an option to qemu-img convert, "-C", to enable (attempting) copy 
> offloading
> explicitly. And mark it incompatible with "-S" and "-c".

Reviewed-by: Kevin Wolf <address@hidden>

Not sure why you made this an RFC only, but I think we absolutely need
this. People are used to using 'qemu-img convert' to compact images and
this would regress with automatic copy offloading.

Do you think we need more discussion?


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