linux-ext4 - Re: ENOSPC returned during writepages

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Message-ID: <20080821153558.GJ16634@mit.edu>
Date:	Thu, 21 Aug 2008 11:35:58 -0400
From:	Theodore Tso <tytso@....edu>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Mingming Cao <cmm@...ibm.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: ENOSPC returned during writepages

On Thu, Aug 21, 2008 at 08:48:15PM +0530, Aneesh Kumar K.V wrote:
> > I have a feeling that we did not try very hard before invalidate the
> > dirty page which fail to map to disks. Perhaps we should try a few more
> > times before give up. Also in that case, perhaps we should turn off
> > delalloc fs wide, so the new writers won't take the subsequently made
> > avaible free blocks away from this unlucky delalloc da writepages.
> 
> How do we try hard ? The mballoc already try had to allocate blocks. So I
> am not sure what do we achieve by requesting for block allocation again.

So here's the problem that we face.  If we have a situation where the
disk fills temporarily, but then subsequently space gets freed up, it
would be preferable if the dirty page isn't invalidated, and so
periodically (or perhaps via "there's-free-space-now notifier") we
retry the delayed allocation so we don't lose data during a transient
disk full situation.  But at the same time, we don't want an fsync()
on the entire filesystem, or a umount on the filesystem, to hang
forever.

						- Ted
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