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Re: [Qemu-devel] [PATCH v2 0/1] Don't obey the kernel block device max t

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v2 0/1] Don't obey the kernel block device max transfer len / max segments for raw block devices
Date: Fri, 12 Jul 2019 11:20:06 +0200
User-agent: Mutt/1.11.3 (2019-02-01)

Am 04.07.2019 um 14:43 hat Maxim Levitsky geschrieben:
> Linux block devices, even in O_DIRECT mode don't have any user visible
> limit on transfer size / number of segments, which underlying kernel block 
> device can have.
> The kernel block layer takes care of enforcing these limits by splitting the 
> bios.
> By limiting the transfer sizes, we force qemu to do the splitting itself which
> introduces various overheads.
> It is especially visible in nbd server, where the low max transfer size of the
> underlying device forces us to advertise this over NBD, thus increasing the
> traffic overhead in case of image conversion which benefits from large blocks.
> More information can be found here:
> https://bugzilla.redhat.com/show_bug.cgi?id=1647104
> Tested this with qemu-img convert over nbd and natively and to my surprise,
> even native IO performance improved a bit.

Thanks, applied to the block branch.


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