linux-kernel - Re: [LFS/VM TOPIC] Stable pages while IO (was Wrong DIF guard tag on ext2 write)

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Date:	Mon, 7 Jun 2010 01:37:53 +0200
From:	Jan Kara <jack@...e.cz>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	Jan Kara <jack@...e.cz>,
	Christof Schmitt <christof.schmitt@...ibm.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, lsf10-pc@...ts.linuxfoundation.org,
	Nick Piggin <npiggin@...e.de>,
	Al Viro <viro@...IV.linux.org.uk>,
	Chris Mason <chris.mason@...cle.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Vladislav Bolkhovitin <vst@...b.net>,
	Christoph Hellwig <hch@....de>
Subject: Re: [LFS/VM TOPIC] Stable pages while IO (was Wrong DIF guard tag
 on ext2 write)

On Sun 06-06-10 12:35:03, Boaz Harrosh wrote:
> On 06/04/2010 07:23 PM, Jan Kara wrote:
> > On Thu 03-06-10 19:09:52, Boaz Harrosh wrote:
> >> [Topic]
> >> How to not let pages change while in IO
> >>
> >> [Abstract]
> >> As seen in a long thread on the fsdvel scsi mailing lists. Lots of
> >> people have headaches and sleep less nights because individual pages
> >> can change while in IO and/or DMA. Though each one as slightly different
> >> needs, the mechanics look to be the same.
> 
> >   Hmm, I don't think it's really about "how to not let pages change" - that
> > is doable by using wait_on_page_writeback() in ->page_mkwrite and
> > ->write_begin. I think the discussion is more about whether we should do it
> > or whether we should rechecksum and resubmit IO in case of checksum failure
> > as Nick proposed...
> > 
> > 								Honza
> 
> I have hijacked the DIF threads but, No, my proposal is for a general
> toolset that could be used for all the above as well as DIF if needed.
> 
> Surly even with DIF the keep-constant vs retransmit is a matter of
> machine+link speed multiply by faulting work loads. So there might be
> situations where an admin wants to choose.
> 
> With other none checksum fixtures, like RAID5/MIRROR this is not always
> an option and it becomes keep-constant vs copy. (That is complete
> workload copy). So for these setups the option is clear. No?
  Is it? You can have enough CPU / memory bandwidth to do the copying while
you need not be comfortable with a thread blocking until IO is finished
when it tries to do a rewrite...

> I'm glad that you think it is easy/doable to implement. And I'll surly
> test your above receipt. Do you think it would be acceptable as a generic
> per-sb tunable.  So for instance an ext3 over RAID5 could turn this on
> and eliminate the data copy?
  Yes, that would be useful. At least so that one can get real performance
numbers...

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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