Re: [PATCH v2 0/7] btrfs-progs: Support for BG_TREE feature

Qu Wenruo
2019-10-22T00:49:53Z

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On 2019/10/21 下午11:44, David Sterba wrote:
> On Sat, Oct 19, 2019 at 08:04:51AM +0800, Qu Wenruo wrote:
>> That's wonderful.
>> Although I guess my patchset should provide the hint of where to modify
>> the code, since there are only a limited number of places we modify
>> block group item.
>
> I indeed started at your patchset and grepped fro BG_TREE, adding
> another branch.
>
>>> We agree on the point that the block group items must be packed. The key
>>> approach should move the new BGI to the beginning, ie. key type is
>>> smaller than anything that appears in the extent tree. I chose 100 for
>>> the prototype, it could change.
>>>
>>> To keep changes to minimum, the new BGI uses the same block group item,
>>> so the only difference then becomes how we search for the items.
>>
>> If we're introducing new block group item, I hope to do a minor change.
>>
>> Remove the chunk_objectid member, or even flags. to make it more
>> compact. So that you can make the BGI subtree even smaller.
>
> Yeah that can be done.
>
>> But I guess since you don't want to modify the BGI structure, and keep
>> the code modification minimal, it may not be a good idea right now.
>
> As long as the changes are bearable, eg. a minor refactoring of block
> group access (the cache.key serving a as offset and length) is fine. And
> if we can make the b-tree item more then let's do it.
>
>>> Packing of the items is done by swapping the key objectid and offset.
>>>
>>> Normal BGI has bg.start == key.objectid and bg.length == key.offset. As
>>> the objectid is the thing that scatters the items all over the tree.
>>>
>>> So the new BGI has bg.length == key.objectid and bg.start == key.offset.
>>> As most of block groups are of same size, or from a small set, they're
>>> packed.
>>
>> That doesn't look optimized enough.
>>
>> bg.length can be at 1G, that means if extents starts below 1G can still
>> be before BGIs.
>
> This shold not be a big problem, the items are still grouped togethers.
> Mkfs does 8M, we can have 256M or 1G. On average there could be several
> packed groups, which I think is fine and the estimated overhead would be
> a few more seeks.
>
>> I believe we should have a fixed objectid for this new BGIs, so that
>> they are ensured to be at the beginning of extent tree.
>
> That was my idea too, but that also requires to add one more member to
> the item to track the length. Currently the key is saves the bytes. With
> the proposed changes to drop chunk_objectid, the overall size of BGI
> would not increase so this still sounds ok. And all the problems with
> searching would go away.
>
>>> The nice thing is that a lot of code can be shared between BGI and new
>>> BGI, just needs some care with searches, inserts and search key
>>> advances.
>>
>> Exactly, but since we're introducing a new key type, I prefer to perfect
>> it. Not only change the key, but also the block group item structure to
>> make it more compact.
>>
>> Although from the design aspect, it looks like BG tree along with new
>> BGI would be the best design.
>>
>> New BG key goes (bg start, NEW BGI TYPE, used) no data. It would provide
>> the most compact on-disk format.
>
> That's very compact. Given the 'bg start' we can't use the same for the
> extent tree item.
>
>>> This would be easy with the bg_tree, because we'd know that all items in
>>> the tree are just the block group items. Some sort of enumeration could
>>> work for bg_key too, but I don't have something solid.
>>
>> Why not fixed objectid for BGI and just ignore the bg.len part?
>
> I wanted to avoid storing a useless number, it costs 8 bytes per item,
> and the simple swap of objectid/offset was first idea how to avoid it.
>
>> We have chunk<->BGI verification code, no bg.len is not a problem at
>> all, we can still make sure chunk<->bg is 1:1 mapped and still verify
>> the bg.used.
>
> This is all great, sure, and makes the extensions easy. Let's try to
> work out best for each approach we have so far. Having a separate tree
> for block groups may open some future options.

Great, I'll explore the most compact key only method with BG_TREE.

And maybe also try the fixed key objectid solution, just dropping the
bg.length, while keep the current BGI structure.

Thanks,
Qu

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