Subject: kern/34293: vnd deadlocks on I/O buffers
To: None <email@example.com, firstname.lastname@example.org,>
From: None <email@example.com>
Date: 08/26/2006 22:00:01
>Synopsis: vnd deadlocks on I/O buffers
>Arrival-Date: Sat Aug 26 22:00:00 +0000 2006
>Originator: Michael van Elst
>Release: NetBSD 4.99.1
Michael van Elst
"A potential Snark may lurk in every tree."
System: NetBSD pepew 4.99.1 NetBSD 4.99.1 (PEPEW) #32: Sat Aug 26 23:34:55 CEST 2006 src@pepew:/usr/obj/usr/src/sys/arch/i386/compile/PEPEW i386
When writing to a vnd device faster than the underlying
hardware can handle for a sufficient long enough time
then the system will exhaust all I/O buffers. This is
expected. However, the vnd device then deadlocks and the
queued I/O will never complete.
The reason is that vnd uses nested I/O buffers. In order
to process a queued I/O request it has to allocate yet
another I/O buffer. When all I/O buffers have already
been exhausted it therfore waits forever.
Create a vnd device for a large (e.g. 10GB) file on the local disk.
Write lots of data to the vnd device (e.g. mkfs.ext2).
Watch the system deadlock on I/O buffers. The writing process
and the vnd kernel thread are waiting on "getnewbuf".
I have changed the system to use a separate pool for nested I/O
buffers. This works, but there are probably better ways to
solve the problem.