linux-ext4 - Re: [EXT2] Discard unused sectors

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Message-ID: <20080815181847.GF6511@skywalker>
Date:	Fri, 15 Aug 2008 23:48:47 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Theodore Tso <tytso@....edu>
Cc:	David Woodhouse <dwmw2@...radead.org>, linux-ext4@...r.kernel.org
Subject: Re: [EXT2] Discard unused sectors

On Fri, Aug 15, 2008 at 08:02:35AM -0400, Theodore Tso wrote:
> On Thu, Aug 14, 2008 at 10:05:48AM +0100, David Woodhouse wrote:
> > I'm not sure how to do this for ext[34]. The sb_issue_discard() function
> > issues its requests as a soft barrier, because for naïve callers it
> > needs to ensure that the discard happens _before_ any subsequent writes
> > to the same sectors (if they get reallocated immediately).
> > 
> > But ext[34] can probably do better than that, and submit the discard
> > requests _without_ barriers of their own. If someone with a bit more
> > clue does it, that is.
> 
> It's worse than this.  We can't call sb_issue_discard() until the
> transaction commits, since if we crash before the commit, the undelete
> will not have happened.  (The block/inode bitmaps, inode table,
> et. al., aren't allowed to go out to disk until the transaction
> commit, and similarly, those sectors aren't allowed to get reused
> until the commit happens, as well.)  
> 
> This is going to be true of any filesystem which is doing journaling.
> What makes life a bit more difficult for ext4 is that we are doing
> physical block journaling, so we're not keeping track which blocks are
> getting discarded.  (In contrast, systems that do logical journaling
> are keeping track of specific lists of blocks that are getting freed,
> since that's what they write to the journal.)  This means we'll have
> to keep our own in-memory list of extents for which we should call
> sb_issue_discard() when the transaction finally commits.  So this is
> something that we would have to track in the jbd/jbd2 layer, hanging
> off of the transaction structure.  If we do this right, it will also
> be what OCFS2 can use too (since it uses the jbd layer as well.)

Doesn't both ext3 and ext4 do this via 
ext4_journal_get_undo_access and ext4_mb_free_metadata ?. We actually
wait for the transaction to commit to free the meta-data blocks used by the
transaction

-aneesh
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