Re: [QUESTION] about the freelist allocator in XFS

[Kaho Ng <ngkaho1234@gmail.com>]

Hmm, wouldn't xfs_alloc_ag_vextent_size() first remove the free extent
record, and insert a new extent record into the freespace by-size
btree if the found free extent record is longer than args->maxlen?

On Fri, Jul 8, 2016 at 11:47 AM, Dave Chinner <david@fromorbit.com> wrote:
> On Fri, Jul 08, 2016 at 10:26:33AM +0800, Kaho Ng wrote:
>> Hmm, wouldn't xfs_alloc_ag_vextent_size() first remove the free extent
>> record, and insert a new extent record into the freespace by-size
>> btree if the found free extent record is longer than args->maxlen?
>
> Please reply to the list, not privately.
>
> -Dave.
>
>>
>> On Fri, Jul 8, 2016 at 6:28 AM, Dave Chinner <david@fromorbit.com> wrote:
>> > On Thu, Jul 07, 2016 at 07:01:35PM +0800, Kaho Ng wrote:
>> >> I am trying to investigate how freelist allocator in xfs interacts
>> >> with freespace B+Tree allocator.
>> >> First I prepared a patch
>> >> <https://gist.github.com/22ffca35929e67c08759b057779b7566> on
>> >> linux-source/fs/xfs/libxfs/xfs_alloc.c to print debugging messages
>> >> (The kernel version used is linux-3.10.0-327.22.2.el7).
>> > ......
>> >> When reading the log output
>> >> <https://gist.github.com/890076405e1c13c0a952a579e25e6afe> , I
>> >> realised that there is no B+Tree split
>> >> triggered by xfs_alloc_fix_freelist() when calling xfs_free_extent().
>> >> Isn't B+Tree split possible in by-size B+Tree even when truncating a
>> >> longer freespace record to shorter one? But what I found in the log is
>> >> only a few tree shrinks... And when reading the source code of
>> >> freespace allocator I found that a B+Tree growth in this case is
>> >> impossible at least...
>> >
>> > args->isfl doesn't mean what you think it means.
>> >
>> > args->isfl is only set when moving blocks from the freespace btree
>> > to the AGFL, which only occurs when a previous operation allocated a
>> > new freespace btree block and depleted the current freelist. i.e.
>> > "AG Free List" != "AG freespace btree" - they are different
>> > structures on disk...
>> >
>> > And when you consider that a freelist refill can only remove records
>> > from the the freespace btree, it's should be clear that a btree
>> > split won't occur during a freelist refill...
>> >
>> > Cheers,
>> >
>> > Dave.
>> > --
>> > Dave Chinner
>> > david@fromorbit.com
>>
>
> --
> Dave Chinner
> david@fromorbit.com

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