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Date:	Tue, 22 Jan 2008 19:16:39 +1100
From:	David Chinner <dgc@....com>
To:	David Chinner <dgc@....com>,
	Valerie Henson <val@...consulting.com>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger@...sterfs.com>,
	Ric Wheeler <ric@....com>
Subject: Re: [RFC] Parallelize IO for e2fsck

On Tue, Jan 22, 2008 at 12:05:11AM -0700, Andreas Dilger wrote:
> On Jan 22, 2008  14:38 +1100, David Chinner wrote:
> > On Mon, Jan 21, 2008 at 04:00:41PM -0700, Andreas Dilger wrote:
> > > I discussed this with Ted at one point also.  This is a generic problem,
> > > not just for readahead, because "fsck" can run multiple e2fsck in parallel
> > > and in case of many large filesystems on a single node this can cause
> > > memory usage problems also.
> > > 
> > > What I was proposing is that "fsck.{fstype}" be modified to return an
> > > estimated minimum amount of memory needed, and some "desired" amount of
> > > memory (i.e. readahead) to fsck the filesystem, using some parameter like
> > > "fsck.{fstype} --report-memory-needed /dev/XXX".  If this does not
> > > return the output in the expected format, or returns an error then fsck
> > > will assume some amount of memory based on the device size and continue
> > > as it does today.
> > 
> > And while fsck is running, some other program runs that uses
> > memory and blows your carefully calculated paramters to smithereens?
> 
> Well, fsck has a rather restricted working environment, because it is
> run before most other processes start (i.e. single-user mode).  For fsck
> initiated by an admin in other runlevels the admin would need to specify
> the upper limit of memory usage.  My proposal was only for the single-user
> fsck at boot time.

The simple case. ;)

Because XFS has shutdown features, it's not uncommon to hear about
people running xfs_repair on an otherwise live system. e.g. XFS
detects a corrupted block, shuts down the filesystem, the admin
unmounts it, runs xfs_repair, puts it back online. meanwhile, all
the other filesystems and users continue unaffected. In this use
case, getting feedback about memory usage is, IMO, very worthwhile.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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