From: Boaz Harrosh <bharrosh@panasas.com>
Subject: Re: [RFC PATCH 1/2] bdi: Create a flag to indicate that a backing
 device needs stable page writes
Date: Wed, 31 Oct 2012 16:12:53 -0700
Message-ID: <5091B075.7030608@panasas.com>
References: <20121026101909.GB19617@blackbox.djwong.org> <20121027013524.GA19591@blackbox.djwong.org> <yq1ehkg8us1.fsf@sermon.lab.mkp.net> <20121030154844.1898f068@notabene.brown> <yq1a9v4884i.fsf@sermon.lab.mkp.net> <20121030201424.GD19559@blackbox.djwong.org> <20121031091441.5fc6b412@notabene.brown> <20121031085604.GC19591@blackbox.djwong.org> <20121031115614.GC18424@quack.suse.cz> <20121031193652.GF19591@blackbox.djwong.org>
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Cc: Jan Kara <jack@suse.cz>, NeilBrown <neilb@suse.de>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	"Theodore Ts'o" <tytso@mit.edu>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
In-Reply-To: <20121031193652.GF19591@blackbox.djwong.org>
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On 10/31/2012 12:36 PM, Darrick J. Wong wrote:
> On Wed, Oct 31, 2012 at 12:56:14PM +0100, Jan Kara wrote:
<snip>
>> You are right that we need a mechanism to push the flags from the devices
>> through various storage layers up into the bdi filesystem sees to make
>> things reliable.
> 
> md/dm will call blk_integrity_register, so pushing the "stable page writes
> required" flag through the various layers is already taken care of.  If devices
> and filesystems can both indicate that they want stable page writes,  I'll have
> to keep track of however many users there are.
> 

Please note again the iscsi case. Say the admin defined half of an md iscsi devices
with data_integrity and half without.

For me I would like an OR. If any underline device needs "stable pages" they all
get them.

Please also provide - or how easy is to make an API - for the like of iscsi that
given a request_queue, set the BDI's "stable pages" on. Something like:

/* stable_pages can only be turned on never off */
blk_set_stable_pages(struct request_queue); 

> It does seem like less work to fix all the filesystems than to dork around with
> another flag.

Sure if that is possible, that will be perfect, then I do not need to keep
the old "unstable pages" code around at all.

Thanks for working on this
Boaz