Message-ID: <20100623233759.GE7058@shareable.org>
Date:	Thu, 24 Jun 2010 00:37:59 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Edward Shishkin <edward.shishkin@...il.com>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Edward Shishkin <edward@...hat.com>,
	Mat <jackdachef@...il.com>, LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, Ric Wheeler <rwheeler@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	The development of BTRFS <linux-btrfs@...r.kernel.org>
Subject: Re: Balancing leaves when walking from top to down (was Btrfs:...)

Edward Shishkin wrote:
> Chris Mason wrote:
> >On Tue, Jun 22, 2010 at 04:12:57PM +0200, Edward Shishkin wrote:
> >  
> >>Chris Mason wrote:
> >>    
> >>>On Mon, Jun 21, 2010 at 09:15:28AM -0400, Chris Mason wrote:
> >>>      
> >>>>I'll reproduce from your test case and provide a fix.  mount -o
> >>>>max_inline=1500 would give us 50% usage in the worst case
> >>>>        
> >>This is a very strange statement: how did you calculate this lower bound?
> >>    
> >
> >We want room for the extent and the inode item and the inode backref.
> >It's a rough number that leaves some extra room.  But even with a 2K
> >item we're getting very close to 50% usage of the metadata area.
> >
> >  
> >>>>(minus the
> >>>>balancing bug you hit).
> >>>>        
> >>>Ok, the balancing bug was interesting.  What happens is we create all
> >>>the inodes and directory items nicely packed into the leaves.
> >>>
> >>>Then delayed allocation kicks in and starts inserting the big fat inline
> >>>extents.  This often forces the balancing code to split a leaf twice in
> >>>order to make room for the new inline extent.  The double split code
> >>>wasn't balancing the bits that were left over on either side of our
> >>>desired slot.
> >>>
> >>>The patch below fixes the problem for me, reducing the number of leaves
> >>>that have more than 2K of free space down from 6% of the total to about
> >>>74 leaves.  It could be zero, but the balancing code doesn't push
> >>>items around if our leaf is in the first or last slot of the parent
> >>>node (complexity vs benefit tradeoff).
> >>>      
> >>Nevertheless, I see leaves, which are not in the first or last slot,
> >>but mergeable with their neighbors (test case is the same):
> >>    
> >
> >Correct, but it was in the first or last slot when it was balanced (I
> >confirmed this with printk).
> >
> >Then the higher layers were balanced and our leaves were no longer in
> >the first/last slot.  We don't rebalance leaves when we balance level 1.
> >
> >  
> >>leaf 269557760 items 22 free space 2323 generation 25 owner 2
> >>fs uuid 614fb921-cfa9-403d-9feb-940021e72382
> >>chunk uuid b1674882-a445-45f9-b250-0985e483d231
> >>
> >>leaf 280027136 items 18 free space 2627 generation 25 owner 2
> >>fs uuid 614fb921-cfa9-403d-9feb-940021e72382
> >>chunk uuid b1674882-a445-45f9-b250-0985e483d231
> >>
> >>node 269549568 level 1 items 60 free 61 generation 25 owner 2
> >>fs uuid 614fb921-cfa9-403d-9feb-940021e72382
> >>chunk uuid b1674882-a445-45f9-b250-0985e483d231
> >>       key (175812608 EXTENT_ITEM 4096) block 175828992 (42927) gen 15
> >>       key (176025600 EXTENT_ITEM 4096) block 176111616 (42996) gen 15
> >>       key (176238592 EXTENT_ITEM 4096) block 176300032 (43042) gen 15
> >>       key (176451584 EXTENT_ITEM 4096) block 216248320 (52795) gen 17
> >>       key (176672768 EXTENT_ITEM 4096) block 216236032 (52792) gen 17
> >>       key (176783360 EXTENT_ITEM 4096) block 216252416 (52796) gen 17
> >>       key (176955392 EXTENT_ITEM 4096) block 138854400 (33900) gen 25
> >>       key (177131520 EXTENT_ITEM 4096) block 280289280 (68430) gen 25
> >>       key (177348608 EXTENT_ITEM 4096) block 280285184 (68429) gen 25
> >>       key (177561600 EXTENT_ITEM 4096) block 269557760 (65810) gen 25
> >>       key (177795072 EXTENT_ITEM 4096) block 280027136 (68366) gen 25
> >>       key (178008064 EXTENT_ITEM 4096) block 280064000 (68375) gen 25
> >>       key (178233344 EXTENT_ITEM 4096) block 285061120 (69595) gen 25
> >>       key (178442240 EXTENT_ITEM 4096) block 178442240 (43565) gen 16
> >>       key (178655232 EXTENT_ITEM 4096) block 178655232 (43617) gen 16
> >>       key (178868224 EXTENT_ITEM 4096) block 178868224 (43669) gen 16
> >>[...]
> >>
> >>    
> >>>With the patch, I'm able to create 106894 files (2K each) on a 1GB FS.
> >>>That doesn't sound like a huge improvement, but the default from
> >>>mkfs.btrfs is to duplicate metadata.  After duplication, that's about
> >>>417MB or about 40% of the overall space.
> >>>
> >>>When you factor in the space that we reserve to avoid exploding on
> >>>enospc and the space that we have allocated for data extents, that's not
> >>>a very surprising number.
> >>>
> >>>I'm still putting this patch through more testing, the double split code
> >>>is used in some difficult corners and I need to make sure I've tried
> >>>all of them.
> >>>      
> >>Chris, for the further code review we need documents, which reflect
> >>the original ideas of the balancing, etc. Could you please provide them?
> >>Obviously, I can not do it instead of you, as I don't know those ideas.
> >>
> >>    
> >
> >Which part are you most interested in?
> >  
> 
> Balancing.
> 
> What conditions do we want to provide?
> Why won't we get utilization slump in future?
> How do we need to fix things when something goes wrong?
> Whereas in accordance with the single existing paper Btrfs
> design looks really broken, because:

> 1. the balancing condition n <= number_of_keys <= 2n+1
>   is not satisfied (indeed, when number_of_keys is 1
>   we have 1 <= 2n+1 -- false);

That doesn't matter.  It is not necessary (or desirable) to follow the
classical B-tree algorithms to the letter to get sufficiently good
properties.  B-trees are quite robust to changing the details, as long
as you follow the generalised principles which make them work.

(It is entirely possible that btrfs does not follow those principles
though :-)

All that matters to guarantee utilisation and algorithmic performance
is to do enough merging on deletions, and avoid doing too many splits,
to ensure that an amount X% of all used blocks (except the root) are
at least Y% full.

Y is 50 and X is 100 in your description, but other numbers work too.
Reducing X slightly improves utilisation in common filesystem
scenarios where many blocks are filled in ascending key sequences.
(But delayed allocation/writing, like btrfs does, is another way to
achieve a similar result.)  Increasing Y improves minimum utilisation,
which seems like a good thing to demand from a filesystem - for the
actual data, anyway.

>From what's been said in this thread, I'm getting the impression that
btrfs does not guarantee a particular value of X, but rather depends
heuristically on typical behaviour to give an equivalent outcome, and
that that does not always work in some pathological cases.  And also
that Y is/was not guaranteed, but that may be quite possibly an
ordinary bug and might have been fixed by Chris during this thread.

I must admit that only getting 200GB of 2kB files into a formatted 1GB
partition before it says ENOSPC - and that's after Chris's recent
tweak - is a bit disappointing.  A little over 50% loss for 2kB files
is quite reasonable - most filesystems would do that just due to
padding to the next 4kB (the block size), and about 50% wastage in a
B-tree is fair enough (at least if there is no sane pattern to the
file's creation), as guaranteeing higher utilisation has a performance
penalty.  But with the delayed allocation, I'd hope for better packing
than the worst case, and I'd expect even the worst case to be better
than 20% available for data.

Losing 80% of the space to utilisation loss and metadata duplication
is disappointing (to me), and a bit suprising, given that one of the
purposes of inline extents is to pack small files more efficiently
than if they were padded to the block size.

> 2. the trees mentioned in this paper don't allow keys of
>   variable length.

Inline extents aren't the same as variable-length keys, because they
aren't sorted or searched by content.  I showed in an earlier reply to
you how to show that variable length *values*[1] don't cause
unbalancing provided the splits are put in acceptable places.  But to
make sense of that, you do have to grok the generalising of B-trees
beyond the original algorithms.

[1] Think of the B-tree variants which contain (key,value) pairings
where "value" is not ordered, rather than the type which contain just
"keys" which are fully ordered.

Are there any true (fully ordered) variable-length keys in btrfs?

-- Jamie
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists