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Re: [PATCH 0/2] block: add logging facility for long standing IO request

From: Stefan Hajnoczi
Subject: Re: [PATCH 0/2] block: add logging facility for long standing IO requests
Date: Fri, 31 Jul 2020 11:25:31 +0100

On Fri, Jul 10, 2020 at 08:27:09PM +0300, Denis V. Lunev wrote:
> There are severe delays with IO requests processing if QEMU is running in
> virtual machine or over software defined storage. Such delays potentially
> results in unpredictable guest behavior. For example, guests over IDE or
> SATA drive could remount filesystem read-only if write is performed
> longer than 10 seconds.
> Such reports are very complex to process. Some good starting point for this
> seems quite reasonable. This patch provides one. It adds logging of such
> potentially dangerous long IO operations.
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>

If I understand correctly this only reports completed I/Os, so if the
host hasn't given up on an I/O request yet then QEMU will not report it
is taking a long time. In the meantime the guest could start printing
timeout errors.

I think this patch series is good as it is. In the future maybe a QMP
command that lists in-flight I/O requests would be nice. That helps
when troubleshooting I/Os that are hung.


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