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

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Message-ID: <4C0CAE14.1020205@panasas.com>
Date:	Mon, 07 Jun 2010 11:30:12 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Jan Kara <jack@...e.cz>
CC:	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 06/07/2010 02:37 AM, Jan Kara wrote:
>> 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

Thanks Jan.
You have helped me tremendously. I think I can begin to understand now what I
need to do.

With the workloads I need (HPC), every cycle/memory counts and that the app
waits for a rewrite is a good thing, which reminds me that I would want to trace
that case so applications could be fixed, tuned.

I do understand that for a desktop, that might be just the opposite, so testing
is important. Perhaps I'll need help in instrumenting all this.

Thanks
Boaz
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