linux-kernel - Re: [dm-devel] REQUEST for new 'topology' metrics to be moved out of the 'queue' sysfs directory.

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Date:	Fri, 26 Jun 2009 10:48:03 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Neil Brown <neilb@...e.de>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	"Mike Snitzer" <snitzer@...hat.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Alasdair G Kergon" <agk@...hat.com>, jens.axboe@...cle.com,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	"device-mapper development" <dm-devel@...hat.com>
Subject: Re: [dm-devel] REQUEST for new 'topology' metrics to be moved out  of the 'queue' sysfs directory.

>>>>> "Neil" == Neil Brown <neilb@...e.de> writes:

Neil> Providing the fields are clearly and unambiguously documented so
Neil> that it I can use the documentation to verify the implementation
Neil> (in md at least), I will be satisfied.

The current sysfs documentation says:

/sys/block/<disk>/queue/minimum_io_size:
[...] For RAID arrays it is often the stripe chunk size.

/sys/block/<disk>/queue/optimal_io_size:
[...] For RAID devices it is usually the stripe width or the internal
block size.

The latter should be "internal track size".  But in the context of MD I
think those two definitions are crystal clear.


As far as making the application of these values more obvious I propose
the following:

What:		/sys/block/<disk>/queue/minimum_io_size
Date:		April 2009
Contact:	Martin K. Petersen <martin.petersen@...cle.com>
Description:
		Storage devices may report a granularity or minimum I/O
		size which is the device's preferred unit of I/O.
		Requests smaller than this may incur a significant
		performance penalty.

		For disk drives this value corresponds to the physical
		block size. For RAID devices it is usually the stripe
		chunk size.

		A properly aligned multiple of minimum_io_size is the
		preferred request size for workloads where a high number
		of I/O operations is desired.


What:		/sys/block/<disk>/queue/optimal_io_size
Date:		April 2009
Contact:	Martin K. Petersen <martin.petersen@...cle.com>
Description:
		Storage devices may report an optimal transfer length or
		streaming I/O size which is the device's preferred unit
		of sustained I/O.  This value is a multiple of the
		device's minimum_io_size.

		optimal_io_size is rarely reported for disk drives. For
		RAID devices it is usually the stripe width or the
		internal track size.

		A properly aligned multiple of optimal_io_size is the
		preferred request size for workloads where sustained
		throughput is desired.

After contemplating for a bit I think I prefer to keep them I/O
direction agnostic.  Granted, the potential penalties mostly apply to
writes.  But I think the application of the values apply to reads as
well.  They will in a hw RAID context for sure.


Neil> I'm looking forward to seeing how you justify the name
Neil> "physical_block_size" in a way the encompasses possibilities like
Neil> a device that stripes over a heterogeneous set of disk drives ;-)

I explained that in my mails yesterday.  But that is of no concern to
MD.

-- 
Martin K. Petersen	Oracle Linux Engineering
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