linux-kernel - Re: [vm] writing to UDF DVD+RW (/dev/sr0) while under memory pressure: box ==> doorstop

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Date:	Tue, 08 Jan 2008 16:21:53 +0100
From:	Mike Galbraith <efault@....de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [vm] writing to UDF DVD+RW (/dev/sr0) while under memory
	pressure: box ==> doorstop


On Tue, 2008-01-08 at 03:38 -0800, Andrew Morton wrote:
> On Tue, 08 Jan 2008 12:05:16 +0100 Mike Galbraith <efault@....de> wrote:

> > Seeks on DVD drives are apparently far FAR more horrible that I ever
> > imagined, so writing to a filesystem on a DVD+RW in effect constitutes
> > the stupid thing I was worried about above.  Make an image, populate it,
> > write it instead... sure to be a truckload faster, and due to the VM
> > connection, much safer too.  Oh well.
> > 
> 
> Well.  From your earlier trace it appeared that something was causing
> the filesystem to perform synchronous inode writes - sync_dirty_buffer() was
> called.
> 
> This will cause many more seeks than would occur if we were doing full
> delayed writing, with obvious throughput implications.

Yes, with UDF, the IO was _incredibly_ slow.  With ext2, it was better,
though still very bad.  I tested with that other OS, and it gets ~same
throughput with UDF as I got with ext2 (ick).

UDF does udf_clear_inode() -> write_inode_now(inode, 1)

I suppose I could try write_inode_now(inode, 0).  Might unstick the box.

	-Mike

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