linux-kernel - Re: [dm-devel] [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush

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Message-ID: <4C72A6B2.4070900@redhat.com>
Date:	Mon, 23 Aug 2010 12:49:54 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>, "tytso@....edu" <tytso@....edu>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	"jack@...e.cz" <jack@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"swhiteho@...hat.com" <swhiteho@...hat.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"James.Bottomley@...e.de" <James.Bottomley@...e.de>,
	"konishi.ryusuke@....ntt.co.jp" <konishi.ryusuke@....ntt.co.jp>,
	Tejun Heo <tj@...nel.org>, "vst@...b.net" <vst@...b.net>,
	Christoph Hellwig <hch@....de>,
	"chris.mason@...cle.com" <chris.mason@...cle.com>
Subject: Re: [dm-devel] [PATCHSET block#for-2.6.36-post] block: replace barrier
 with sequenced flush

On 08/23/2010 12:45 PM, Sergey Vlasov wrote:
> On Mon, Aug 23, 2010 at 11:19:13AM -0400, Ric Wheeler wrote:
> [...]
>> (2) hardware raid cards with internal buffer memory and on-card battery backup
>> (they sit in your server, disks sit in jbod like expansion shelves). These are
>> fine if the drives in those shelves have write cache disabled.
>
> Actually some of such cards keep write cache on the drives enabled and
> issue FLUSH CACHE commands to the drives.  E.g., 3ware 9690SA behaves
> like this at least with SATA drives (the FLUSH CACHE commands can be
> seen after enabling performance monitoring - they often end up in the
> "10 commands having the largest latency" table).  This can actually be
> safe if the card waits for the FLUSH CACHE completion before making
> the write cache data in its battery-backed memory available for reuse
> (and the drive implements the FLUSH CACHE command correctly).

Yes - this is certainly one way to do it. Note that this will not work if the 
card advertises itself as a write through cache (and we end up not sending down 
the SYNC_CACHE commands).

At least one hardware RAID card (I unfortunately cannot mention the brand) did 
not do this command forwarding.

ric

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