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SubjectRe: Btrfs: broken file system design (was Unbound(?) internal fragmentation in Btrfs)
Jamie Lokier wrote:
> Edward Shishkin wrote:
>
>> If you decide to base your file system on some algorithms then please
>> use the original ones from proper academic papers. DO NOT modify the
>> algorithms in solitude: this is very fragile thing! All such
>> modifications must be reviewed by specialists in the theory of
>> algorithms. Such review can be done in various scientific magazines of
>> proper level.
>>
>> Personally I don't see any way to improve the situation with Btrfs
>> except full redesigning the last one. If you want to base your file
>> system on the paper of Ohad Rodeh, then please, use *exactly* the
>> Bayer's B-trees that he refers to. That said, make sure that all
>> records you put to the tree has equal length and all non-root nodes of
>> your tree are at least half filled.
>>
>
> First, thanks Edward for identifying a specific problem with the
> current btrfs implementation.
>

Hello Jamie.

> I've studied modified B-trees quite a lot and know enough to be sure
> that they are quite robust when you modify them in all sorts of ways.
>

Which property is robust?

> Moreover, you are incorrect to say there's an intrinsic algorithmic
> problem with variable-length records. It is not true; if Knuth said
> so, Knuth was mistaken.
>

I didn't say about intrinsic algorithmic problems :)
I just repeat (after Knuth et al) that B-trees with variable-length
records don't
have any sane boundary for internal fragmentation. The common idea is
that if we
don't want Btrfs to be in infinite development stage, then we should
choose some
*sane* strategy (for example the paper of Ohad Rodeh) and strictly
adhere this in
future.

> This is easily shown by modelling variable-length records (key ->
> string) as a range of fixed length records ([key,index] -> byte) and
> apply the standard B-tree algorithms to that, which guarantees
> algorithm properties such as space utilisation and time; then you can
> substitute a "compressed" representation of contiguous index runs,
> which amounts to nothing more than just storing the strings (split
> where the algorithm says to do so) and endpoint indexes , and because
> this compression does not expand (in any way that matters), classic
> algorithmic properties are still guaranteed.
>
> Variable-length keys are a different business. Those are trickier,
> but as far as I know, btrfs doesn't use them.
>
>
>> As to current Btrfs code: *NOT ACK*!!! I don't think we need such
>> "file systems".
>>
>
> Btrfs provides many useful features that other filesystems don't. We
> definitely need it, or something like it. You have identified a bug.
> It's not a corruption bug, but it's definitely a bug, and probably
> affects performance as well as space utilisation.
>
> It is not deep design bug; it is just a result of the packing and
> balancing heuristics.
>

Frankly, I would like to believe to such end, taking into account amount
of my
contributions to the Btrfs project. At least to make sure I didn't do
useless
work..

> If you are still interested, please apply your knowledge of B-tree
> algorithms to understanding why btrfs fails to balance the tree
> sufficiently well,

Because of top-down balancing. It doesn't like "clever" things like
"splitting"
and "merging". Currently top-down works properly only with stupid classic
Bayer's B-trees.

> and then propose a fix.
>

I'll try to help, but I am rather pessimistic here: working out
algorithms is
something, which doesn't like timelines..

> Note that it's not necessarily a problem to have a few nodes with low
> utilisation. Deliberate violation of the classic balancing heuristic
> is often useful for performance.[*]

Ok, let's violate this, but not in the detriment of utilisation:
Internal fragmentation is a horror for file systems, the enemy #1
(which is completely defeated in the last century BTW).

> The problem you've found is only a
> real problem when there are _too many_ nodes with low utilisation.
>

IMHO this is a problem, as we can not estimate number of such nodes.
Do you have any sane upper boundary for this number? I don't know such one.
Maybe I have missed something?

> [*] For example when filling a tree by inserting contiguously
> ascending keys, the classic "split into two when full" heuristic gives
> the worst possible results (50% lost space), and deliberately
> underfilling the most actively updated nodes, which is not permitted
> at all by the classic algorithm, gives denser packing in the end
> (almost zero lost space).

At the end of what? I hope you don't speak about on-line defragmentation?
Can you point me to the paper (if any)?

Thanks!

> It's also faster. The trick is to make
> sure there's just the right number of underfilled nodes...
>
> -- Jamie
> --
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>


--
Edward O. Shishkin
Principal Software Engineer
Red Hat Czech



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