Re: [PATCH 0/4] fat: allocate multiple clusters in fallocate

[OGAWA Hirofumi]

Matheus Alves de Almeida <matheus.aalmeida@xxxxxxxxxxxx> writes:

> Em 2026-10-11 15:55, OGAWA Hirofumi escreveu:
>> Matheus Alves de Almeida <matheus.aalmeida@xxxxxxxxxxxx> writes:
>> 
>>> Hi,
>
>>> This series adds support for allocating multiple clusters in a single
>>> fat_add_clusters() call in fat_fallocate().
>
>>> The multi-cluster TODO in inode.c is not implemented, as doing so would
>>> require speculative preallocation or an iomap conversion.
>
>> Is this to improve the performance of fallocate? If so, what the number
>> by improved bench or such?
>
> Yes, it is. I ran a few benchmarks. Each run used a freshly formatted
> filesystem, with no userspace processes other than init (which is the
> benchmark program in this case).
>
> The numbers below are the arithmetic mean of the samples, with
> improvement calculated as before / after. The times are for the complete
> loop and not an individual fallocate() call.
>
> The benchmarks were run under QEMU/KVM with a virtio-blk device using
> cache=none.
>
> start = now_ns();
> for (unsigned int i = 0; i < count; i++)
>      fallocate(fds[i], FALLOC_FL_KEEP_SIZE, 0, bytes);
> elapsed = now_ns() - start;
>
> FAT32 with 512-byte clusters using count=16 files:
> length    mounted sync?    before    after    improvement    samples
> 256MiB    no               2.328s    0.175s   13.2x          10
> 16MiB     no               149.659ms 14.036ms 10.7x          10
> 16MiB     yes              747.993s  3.445s   217.1x         3*
> 1MiB      yes              46.63s    0.333s   140.1x         10
>
> FAT32 with 512-byte clusters using count=128 files:
> length    mounted sync?    before    after    improvement    samples
> 1MiB      no               90.873ms  4.686ms  19.4x          10
> 16KiB     no               1.287ms   0.241ms  5.3x           10
> 128KiB    yes              47.284s   0.350s   135x           10
> 4KiB      yes              1.98s     0.144s   13.7x          10
>
> FAT32 with 4096-byte clusters using count=16 files:
> length    mounted sync?    before    after    improvement    samples
> 256MiB    no               295.392ms 24.733ms 11.9x          10
> 16MiB     no               20.509ms  1.404ms  14.6x          10
> 16MiB     yes              92.489s   0.348s   265.4x         10
> 1MiB      yes              5.837s    0.048s   121.3x         10
>
> * it would take a lot of time to take 10 samples like the others,
>    so I only took 3 here.
>
> The improvement is large because the old fallocate path allocates one
> cluster at a time, while the new path can allocate an entire chain in
> one call. This is especially visible on synchronous mounts, where the
> new code can batch FAT updates instead of synchronously handling each
> cluster allocation separately.

Thank you. Please add it to the commit log too.

> Thanks,
> Matheus
>
>>> Matheus Alves de Almeida (4):
>>>  fat: remove cluster allocation size limit
>>>  fat: return only first cluster when allocating
>>>  fat: support adding multiple clusters
>>>  fat: allocate multiple clusters in fallocate
>>> 
>>> fs/fat/dir.c    | 20 +++++++++++++++-
>>> fs/fat/fat.h    |  4 ++--
>>> fs/fat/fatent.c | 61 ++++++++++++++++++++++++++++++++-----------------
>>> fs/fat/file.c   | 10 ++++----
>>> fs/fat/inode.c  |  8 +++----
>>> 5 files changed, 69 insertions(+), 34 deletions(-)
>

-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>