Re: Question on slow fallocate

[Wang Yugui <wangyugui@e16-tech.com>]

Hi,

> Hi all,
> 
> When testing PostgreSQL, I found a performance degradation. After some
> investigation, it ultimately reached the attached simple C program and
> turned out that the performance degradation happens on only the xfs
> filesystem (doesn't happen on neither ext3 nor ext4). In short, the
> program alternately does two things to extend a file (1) call
> posix_fallocate() to extend by 8192 bytes and (2) call pwrite() to
> extend by 8192 bytes. If I do only either (1) or (2), the program is
> completed in 2 sec, but if I do (1) and (2) alternatively, it is
> completed in 90 sec.
> 
> $ gcc -o test test.c
> $ time ./test test.1 1
> total   200000
> fallocate       200000
> filewrite       0
> 
> real    0m1.305s
> user    0m0.050s
> sys     0m1.255s
> 
> $ time ./test test.2 2
> total   200000
> fallocate       100000
> filewrite       100000
> 
> real    1m29.222s
> user    0m0.139s
> sys     0m3.139s
> 
> Why does it take so long in the latter case? and are there any
> workaround or configuration changes to deal with it?
> 

I test it on xfs linux 6.1.35 and 6.4-rc7

the result is almost same.

$ time ./test test.1 1
real    0m1.382s

$ time ./test test.2 2
real    0m9.262s

linunx kernel version please.

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2023/06/22