linux-ext4 - Re: [RFC PATCH 1/2] bdi: Create a flag to indicate that a backing device needs stable page writes

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Date:	Wed, 31 Oct 2012 16:12:53 -0700
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
CC:	Jan Kara <jack@...e.cz>, NeilBrown <neilb@...e.de>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	"Theodore Ts'o" <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] bdi: Create a flag to indicate that a backing
 device needs stable page writes

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
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