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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.
>
> 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.
>

This is the point: Which kind of modified B-tree data structure is best
suited?

> 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.
>
> 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.
>

I think this is the most important design question in relation with
filesystems that use some kind of B-trees, which means, if the wrong
modified B-tree as the fundamental data structure was chosen, then this
is a deep design bug.

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

This is a general problem of filesystem design, especially the packing
and balancing heurisitcs, and a special problem of the Btrfs filesystem.
You can't simply say do it in this or that way. That's why another
filesystem uses something exotic like a B*-tree in conjunction with
dancing trees as fundamental data structure, which leads back to the
deep design question/problem/decision/bug/.... And after I followed the
explanations of this exotic B-tree version by the main developer I knew
just right from the start of the development of the Btrfs filesystem
that it wasn't chosen the right modified B-tree data structure, because
it was too simple and too general. And since some days I have the
impression that there wasn't made a design decision at all with the only
exception that there has to be some kind of a B-tree algorithm/data
structure in the Btrfs filesystem.
And I also think that such a deep desgin decision can't simply be
corrected in general (subjective opinion).

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

The found problem is the first problem with the chosen modified B-tree
data structure. I wouldn't call it only a problem in a special case.

> [*] 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). It's also faster. The trick is to make
> sure there's just the right number of underfilled nodes...
>

Yes, but ....
Firstly, maybe you are too focused on the classic B-tree algorithm here.
Secondly, a trick here, a split there, turning off a feature and then?
Then we have complexity at then end, which brings us back to the start,
the design decision.
But if you say there are no deep problems, then I will believe you for now.

> -- Jamie
>
With all the best
Christian Stroetmann




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