Re: [PATCH] Memory management livelock

[Alasdair G Kergon]

On Fri, Oct 03, 2008 at 09:50:17AM -0400, Mikulas Patocka wrote:
> > > LVM does (that is where the bug was discovered). Basically, it scans all
> > > the block devices with direct IO and if someone else does buffered IO on
> > > any device simultaneously, it locks up.
> > Scans all block devices with direct IO? Hmm, why, I wonder? Should
> > really consider using buffered (posix_fadvise to readahead/dropbehind).
> LVM must not allocate any memory when doing IO because it suspends the 
> block device and memory allocation could trigger writeback on the 
> suspended device and deadlock.
> So it preallocates heap and stack, mlockall()s itself and does direct IO.
 
True, but unrelated to the scanning, which LVM performs *prior* to
entering such a state.

We use direct IO while scanning because it's essential all nodes in a
cluster see the same updated version of the data after any node updated
it.

Alasdair
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