linux-ext4 - Re: Help on Implementation of EXT3 type Ordered Mode in EXT4

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Date:	Mon, 15 Feb 2010 16:00:22 +0100
From:	Jan Kara <jack@...e.cz>
To:	Kailas Joshi <kailas.joshi@...il.com>
Cc:	tytso@....edu, linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>,
	Jiaying Zhang <jiayingz@...gle.com>
Subject: Re: Help on Implementation of EXT3 type Ordered Mode in EXT4

On Sat 13-02-10 14:13:17, Kailas Joshi wrote:
> On 13 February 2010 01:37,  <tytso@....edu> wrote:
> > On Fri, Feb 12, 2010 at 08:52:15AM +0530, Kailas Joshi wrote:
> >> Sorry, I didn't understand why processes need to be suspended.
> >> In my scheme, I am issuing magic handle only after locking the current
> >> transaction.  AFAIK after the transaction is locked, it can receive the
> >> block journaling requests for already created handles(in our case, for
> >> already reserved journal space), and the new concurrent requests for
> >> journal_start() will go to the new current transaction. Since, the
> >> credits for locked transaction are fixed (by means of early
> >> reservations) we can know whether journal has enough space for the new
> >> journal_start(). So, as long as journal has enough space available,
> >> new processes need now be stalled.
> >
> > But while you are modifying blocks that need to go into the journal
> > via the locked (old) transaction, it's not safe to start a new
> > transaction and start issuing handles against the new transaction.
> >
> > Just to give one example, suppose we need to update the extent
> > allocation tree for an inode in the locked/committing transaction as
> > the delayed allocation blocks are being resolved --- and in another
> > process, that inode is getting truncated or unlinked, which also needs
> > to modify the extent allocation tree?  Hilarty ensues, unless you use
> > a block all attempts to create a new handle (practically speaking, by
> > blocking all attempts to start a new transaction), until this new
> > delayed allocation resolution phase which you have proposed is
> > complete.
> Okay. So, basically process stalling is unavoidable as we cannot
> modify a buffer data in past transaction after it has been modified in
> current transaction.
> Can we restrict the scope for this blocking? Blocking on
> journal_start() will block all processes even though they are
> operating on mutually exclusive sets of metadata buffers. Can we
> restrict this blocking to allocation/deallocation paths by blocking in
> get_write_access() on specific cases(some condition on buffer)? This
> way, since all files will use commit-time allocation, very few(sync
> and direct-io mode) file operations will be stalled.
  I doubt blocking at buffer-level would be enough. I think that the
journalling layer just does not have enough information for such decisions.
It could be feasible to block on per-inode basis but you'd still have to
give a good thought to modification of filesystem global structures like
bitmaps, superblock, or inode blocks.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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