linux-kernel - Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

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Date:	Fri, 01 Jun 2007 13:07:07 -0400
From:	Valdis.Kletnieks@...edu
To:	Tejun Heo <htejun@...il.com>
Cc:	david@...g.hm, Stefan Bader <Stefan.Bader@...ibm.com>,
	Phillip Susi <psusi@....rr.com>,
	device-mapper development <dm-devel@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	David Chinner <dgc@....com>,
	Andreas Dilger <adilger@...sterfs.com>, ric@....com
Subject: Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

On Fri, 01 Jun 2007 16:16:01 +0900, Tejun Heo said:
> Don't those thingies usually have NV cache or backed by battery such
> that ORDERED_DRAIN is enough?

Probably *most* do, but do you really want to bet the user's data on it?

> The problem is that the interface between the host and a storage device
> (ATA or SCSI) is not built to communicate that kind of information
> (grouped flush, relaxed ordering...).  I think battery backed
> ORDERED_DRAIN combined with fine-grained host queue flush would be
> pretty good.  It doesn't require some fancy new interface which isn't
> gonna be used widely anyway and can achieve most of performance gain if
> the storage plays it smart.

Yes, that would probably be "pretty good".  But how do you get the storage
device to *reliably* tell the truth about what it actually implements? (Consider
the number of devices that downright lie about their implementation of cache
flushing....)

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