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Re: [PATCH] xfs: dummy transactions should not dirty VFS state

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: dummy transactions should not dirty VFS state
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Tue, 3 Aug 2010 09:50:49 -0400
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20100803120728.GA26402@dastard>
References: <1280826605-3506-1-git-send-email-david@xxxxxxxxxxxxx> <20100803095518.GA27470@xxxxxxxxxxxxx> <20100803120728.GA26402@dastard>
User-agent: Mutt/1.5.20 (2009-08-17)
On Tue, Aug 03, 2010 at 10:07:28PM +1000, Dave Chinner wrote:
> Yes, I noticed those things. Especially as I modified the wrong
> one in the first place and realised both need fixing and the
> duplication of code seems completely unnecessary. We should have
> only one copy of this code, not two copies that do slightly
> different things.

Yes, having one copy is much better.

> > For one
> > xfs_commit_dummy_trans doesn't actually commit a synchronous transaction
> > (or rather forces out the log) unless SYNC_WAIT is set,
> 
> I don't think that we really _need_ a non-blocking version - waiting
> for a single sync transaction in xfssyncd once every 36s is hardly
> going to kill performance.

Sounds fair, but it needs documentation in the changelog, and possibly
in the source code as well.

> > in addition
> > to that xfs_fs_log_dummy uses _xfs_trans_alloc, which doesn't get
> > blocked by the filesystem freezing.
> 
> Everything will be clean on a frozen filesystem, so all the current
> code does is block the xfssyncd until the filesytem is
> unfrozen. Given that we can still read everything on the frozen
> filesystem, inode caches can still grow and hence we still need to
> run regular reclaiming. If the xfssyncd is blocked then only memory
> pressure can free up inodes.

That's a reason not to wait.  But given the bugs we had in this area
I'd rather not blindly start the transaction here.

Instead we could check s_frozen manually to no bother even doing
the calls to write the dummy record, plus maybe an assert so that it
trips up for debug builds.

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