linux-kernel - Re: [PATCH] Freeze bdevs when freezing processes.

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Date:	Wed, 25 Oct 2006 10:12:58 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	David Chinner <dgc@....com>
Cc:	Pavel Machek <pavel@....cz>,
	Nigel Cunningham <ncunningham@...uxmail.org>,
	Andrew Morton <akpm@...l.org>,
	LKML <linux-kernel@...r.kernel.org>, xfs@....sgi.com
Subject: Re: [PATCH] Freeze bdevs when freezing processes.

On Wednesday, 25 October 2006 02:13, David Chinner wrote:
> On Tue, Oct 24, 2006 at 11:37:37PM +0200, Pavel Machek wrote:
> > Hi!
> > 
> > > > Do you mean calling sys_sync() after the userspace has been frozen
> > > > may not be sufficient?
> > > 
> > > In most cases it probably is, but sys_sync() doesn't provide any
> > > guarantees that the filesystem is not being used or written to after
> > > it completes. Given that every so often I hear about an XFS filesystem
> > > that was corrupted by suspend, I don't think this is sufficient...
> > 
> > Userspace is frozen. There's noone that can write to the XFS
> > filesystem.
> 
> Sure, no new userspace processes can write data, but what about the
> internal state of the filesystem?
> 
> All a sync guarantees is that the filesystem is consistent when the
> sync returns and XFS provides this guarantee by writing all data and
> ensuring all metadata changes are logged so if a crash occurs it can
> be recovered (which provides the sync guarantee). hence after a
> sys_sync(), XFS will still have lots of dirty metadata that needs to
> be written to disk at some time in the future so the transactions
> can be removed from the log.
> 
> This dirty metadata can be flushed at any time, and the dirty state
> is kept in XFS structures and not always in page structures (think
> multipage metadata buffers).

Are the dirty metadata flushed by a kernel thread?

Greetings,
Rafael


-- 
You never change things by fighting the existing reality.
		R. Buckminster Fuller
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