Re: [PATCH, RFC] Delayed logging of file sizes

[Lachlan McIlroy <lachlan@sgi.com>]

David Chinner wrote:
> On Tue, Nov 27, 2007 at 02:30:25PM +1100, Lachlan McIlroy wrote:
>> David Chinner wrote:
>>> Sorry, i wasn't particularly clear. What I mean was that i_update_core
>>> might disappear completely with the changes I'm making.
>>>
>>> Basically, we have three different methods of marking the inode dirty
>>> at the moment - on  the linux inode (mark_inode_dirty[_sync]()), the
>>> i_update_core = 1 for unlogged changes and logged changes are tracked via 
>>> the
>>> inode log item in the AIL.
>>>
>>> One top of that, we have three different methods of flushing them - one
>> >from the generic code for inodes dirtied by mark_inode_dirty(), one from
>>> xfssyncd for inodes that are only dirtied by setting i_update_core = 1
>>> and the other from the xfsaild when log tail pushing.
>>>
>>> Ideally we should only have a single method for pushing out inodes. The 
>>> first
>>> step to that is tracking the dirty state in a single tree (the inode radix
>>> trees). That means we have to hook ->dirty_inode() to catch all dirtying 
>>> via
>>> mark_inode_dirty[_sync]() and mark the inodes dirty in the radix tree. 
>>> Then we
>>> need to use xfs_mark_inode_dirty_sync() everywhere that we dirty the inode.
>> Don't we already call mark_inode_dirty[_sync]() everywhere we dirty the 
>> inode?
> 
> Maybe. Maybe not. Tell me - does xfs_ichgtime() do the right thing?
> 
> [ I do know the answer to this question and there's a day of kdb tracing
> behind the answer. I wrote a 15 line comment to explain what was going
> on in one of my patches. ]

Are you referring to the !(inode->i_state & I_LOCK) check?
Anyway, since you know the answer why don't you enlighten me?

> 
>>> Once we have all the dirty state in the radix trees we can now get rid of
>>> i_update_core and i_update_size - all they do is mark the inode dirty and
>>> we don't really care about the difference between them(*) - and just use
>>> the dirty bit in the radix tree when necessary.
>> If we want to check if an inode is dirty do we have to look up the dirty
>> bit in the tree or is there some easy way to get it from the inode?
> 
> xfs_inode_clean(ip) is my preferred interface. How that is finally
> implemented will be determined by how this all cleans up and what
> performs the best. If lockless tree lookups don't cause performance
> problems, then there is little reason to keep redundant information
> around.
I can't imagine that a tree lookup (lockless or not) would be faster
than dereferencing fields from the inode.  If keeping the inode's dirty
flags and the ones in the radix tree in sync is an issue then maybe
tree lookups are a performance hit we can live with.

> 
>> By consolidating the different ways of dirtying an inode we lose the ability
>> to know why it is dirty and what action needs to be done to undirty it.
> 
> The only way to undirty an inode is to write it to disk.
True.  I was thinking about what may need to be done before we write it
to disk such as flushing the log but that would just be dependent on
whether the inode is pinned?

> 
>> For example if the inode log item has bits set then we know we have to flush
>> the log otherwise there is no need.  With a general purpose dirty bit we 
> 
> No, if the log item is present and dirty (i.e. inode is in the AIL),
> all it means is that we need to attach a callback to the buffer
> (xfs_iflush_done) when dispatching the I/O to do processing of the
> log item on I/O completion. Whether i_update_core is set or not
> in this case is irrelevant - the log item state overrides that.
> 
>> will
>> have to flush regardless.  And my recent attempt to fix the log replay issue
>> relies on i_update_core to indicate there are unlogged changes - I don't see
>> how that will work with these changes.
> 
> But your changes could not be implemented, either. You can't log the inode
> to clean it - it merely transfers the writeback from one list to
> another.
Could not be implemented?  What was that patch I sent around then?  It was
implemented and it did work - it got XFSQA test 182 to finally pass.  But
sure it wasn't an ideal approach.  I even fixed it so that it didn't dirty
the inode during the transaction.

> 
> So, the cleaner fix is to do this - change the xfs_inode_flush()
> just to unconditionally log the inode and don't do inode writeback *at
> all* from there. That will catch all cases of unlogged changes and leave
> inode writeback to tail-pushing or xfssyncd which can be driven by
> the radix tree.
Huh?  Aren't we trying to minimize the number of transactions we do?  My
changes introduce new transactions but only when we have to.  You're saying
here that we log the inode unconditionally - how is that better?  I'm not
trying to defend my changes here (I don't care how the problem gets fixed)
- I'm just trying to understand why your suggestions are a good idea.

I do like the way it simplifies inode writeback though - a sync would
optionally log all the inodes and then just flush the log and that's it
(I think).

> 
> Basically, if we only ever write state to disk that we've logged,
> then we are home free. That means the only time we should update
> the unlogged fields - timestamps and inode size - is during a
> transaction commit and not during inode writeback. If we do that
> then i_update_core and i_update_size go away completely and the
> only place we need track inode dirty state in XFS is when the
> inodes are in the AIL list.
> 
> Even better: this removes one of the three places where we do inode
> writeback and is a significant step towards:
> 
>>> I'd even like to go as far as a two pass writeback algorithm; pass
>>> one only writes data, and pass two only writes inodes. The second pass
>>> for XFS needs to be delayed until data writeback is complete because of
>>> delalloc and inode size updates redirtying the inode. The current
>>> mechanism means we often do two inode writes for the one data write...
> 
> ----
> 
> What I'm trying to say is that I don't think we can cleanly fix the problem
> with the current structure, so let's not waste time on it. A cleaner
> fix should just fall out a simpler writeback structure.
> 
Fair enough.  I'll wait for the patches.