From: Kevin Wolf <kwolf@redhat.com>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: Fam Zheng <fam@euphon.net>, John Ferlan <jferlan@redhat.com>,
qemu-devel@nongnu.org, qemu-block@nongnu.org,
Max Reitz <mreitz@redhat.com>
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 [thread overview]
Message-ID: <20190712092006.GA4514@dhcp-200-226.str.redhat.com> (raw)
In-Reply-To: <20190704124342.7753-1-mlevitsk@redhat.com>
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.
Kevin
prev parent reply other threads:[~2019-07-12 9:20 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-04 12:43 [Qemu-devel] [PATCH v2 0/1] Don't obey the kernel block device max transfer len / max segments for raw block devices Maxim Levitsky
2019-07-04 12:43 ` [Qemu-devel] [PATCH v2 1/1] raw-posix.c - use max transfer length / max segement count only for SCSI passthrough Maxim Levitsky
2019-07-10 13:43 ` Maxim Levitsky
2019-07-11 10:31 ` [Qemu-devel] [Qemu-block] [PATCH v2 0/1] Don't obey the kernel block device max transfer len / max segments for raw block devices Stefan Hajnoczi
2019-07-12 8:32 ` [Qemu-devel] " Pankaj Gupta
2019-07-12 9:20 ` Kevin Wolf [this message]
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