posix_fallocate(2) performance

Hi,

I've recently got a problem report is that creating fully allocated
volumes in libvirt take a lot of time and resource consuming. Volume
here is represented a single file on a filesystem.

To allocate volumes, libvirt tries various methods, with
posix_fallocate(2) being the first one to try, and manual writing of
zeros being the last option.

I noticed that posix_fallocate(2) is indeed fairly slow. I've
implemented a test application for a quick benchmark (attached; in case
attachment doesn't go through, it's also available on github [1]).

Results I get on FreeBSD 14.3-RELEASE amd64:

$ time ./fallocate test.raw-2 posix_fallocate
safezero_posix_fallocate()
./fallocate test.raw-2 posix_fallocate  0,00s user 12,77s system 9% cpu 2:12,21 total
$ time ./fallocate test.raw-1 slow
safezero_slow()
./fallocate test.raw-1 slow  0,02s user 10,32s system 8% cpu 2:03,66 total
$

Test files are stored on this parititon:

/dev/ada0p2 on / (ufs, NFS exported, local, soft-updates, journaled soft-updates)

In this run posix_fallocate(2) uses even more resources and runs longer
than the writing zeros. It slightly differs from run to run, but
generally these two ways give similar results.

I've tried the same app on Ubuntu Linux:

$ time ./fallocate test.raw-2 posix_fallocate
safezero_posix_fallocate()

real    0m0.023s
user    0m0.000s
sys     0m0.015s
$ time ./fallocate test.raw-1 slow
safezero_slow()

real    0m47.262s
user    0m0.017s
sys     0m18.650s
$

Partition here is:

/dev/sda2 on / type ext4 (rw,relatime,lazytime)

Here, posix_fallocate() is way faster than manual zero'ying.

This makes me wonder:

 - Is posix_fallocate(2) supposed to have similar performance as manual
   writing of zeros?
 - Are there recommendations for UFS tuning to make this type of
   operations faster?
 - Are there maybe better ways to fully allocate a file rather than
   calling posix_fallocate(2)?

Thanks,
Roman


1: https://gist.githubusercontent.com/novel/d5b1fdb54256a9f16f1ee454dd984aaa/raw/22017a3c23658230fdb689967ba54a79d9c9a00b/fallocate.c