linux-kernel - Re: [PATCH] block: export SSD/non-rotational queue flag through sysfs

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Date:	Thu, 15 Jan 2009 14:00:39 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Grant Grundler <grundler@...gle.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Greg Freemyer <greg.freemyer@...il.com>,
	Tejun Heo <htejun@...il.com>, Michael Tokarev <mjt@....msk.ru>,
	Kay Sievers <kay.sievers@...y.org>,
	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>,
	linux-kernel@...r.kernel.org, linux-ide@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] block: export SSD/non-rotational queue flag through
	sysfs

On Thu, 2009-01-15 at 10:55 -0800, Grant Grundler wrote:
> On Thu, Jan 15, 2009 at 8:06 AM, James Bottomley
> <James.Bottomley@...senpartnership.com> wrote:
> ...
> > Right, this is the flash cache idea that's been floating around for a
> > while.  It's even been suggested as a way of avoiding the IDE barrier
> > flush penalties.  I think Seagate went as far as making hybrid drives
> > that were a large flash cache backed by rotational storage.
> 
> "inline" RAM caches  were implemented 10+ years ago for SCSI devices
> to avoid seek *and* rotational delay penalties. These were transperent
> SCSI devices that would cache and prefetch content from disks.
> For the given dataset size, the seek cost was effectively zero. This is
> mostly true today for high-end disk arrays which have large RAM
> front-end's (TB) and very smart data placement strategies (reduced seek).

RAM caches are different from flash caches: they're volatile.
Essentially they're the cause of our barrier flush issues.  The point
about flash caches is they're non volatile.  You can pull power from the
disk as soon as the data is in flash cache; the disc can commit it again
as soon as it powers up.

> Back to the bikeshed painting: I prefer two flags: "AVGREADCOST" and
> "AVGWRITECOST as the flag. This isn't just about seek. Rotational delay
> @7200 RPM is 8.3ms. Making both non-zero implies rotational media.
> Different devices have different read and write characteristics.
> Flash typically has a much higher write cost than read cost.
> Disks (because of WCE) can be the opposite.
> 
> Code can test for zero/nonzero or (preferably) more fine grained.
> e.g. "avgreadcost > 1ms" or "avgwritecost". I'm hoping this test
> can be abstracted into a macro.

Um these really have to be things we can get out of the device at boot
time without effort (as in part of the data the device can give in a
single command).  I'll be shot for increasing boot time so we can work
out these parameters ...

> I'm hoping longterm, the values could be "self tuning" but don't know
> how that might work - e.g. 1 minute avg? 10 minute avg? Cost
> of collecting/maintaining the stats? Feels like a CONFIG option.

CONFIG_SLOW_YOUR_BOOT?

James


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