[Pkg-samba-maint] Bug#911118: mhddfs hangs for a while on writing

[наб]

Control: clone -1 -2
Control: reassign -2 mhddfs 0.1.39+nmu2
Control: severity -2 wishlist
Control: retitle -2 mhddfs: should supoort fallocate

On Sat, Nov 16, 2024 at 08:47:42AM +1300, Andrew Bartlett wrote:
> I've checked our code, if fallocate() fails, we write 32kb chunks.
It also looked like this to me but the falback path wasn't obvious.
(Also, 32k is kinda small IMO but whatever.)

> But
> we won't control how glibc falls back if fallocate is not supported by
> the filesystem. 
Sure, but since you're already implementing the fallback yourself,
you should probably use fallocate() instead of posix_fallocate().

Under glibc, 
  posix_fallocate() does fallocate(2) with the libc fallback if it fails,
  fallocate() just does fallocate(2) and gives you the real error,
so error-detecting after the infallible posix_fallocate() is kinda moot.

Under musl they're both equivalent to fallocate(2).

So while "changing how glibc falls back" is definitely out of scope,
IMO "using a fallible interface to fall back from" could be in scope?

This alone would drop the I/Os issued 8-fold, bumping the buffer
to something more reasonable in 2024 would make it even faster.

> This isn't Samba, perhaps try libc6 or put the bug back to MHDDFS?
I find the belaboured exposition in glibc's posix_fallocate.c convincing,
but MHDDFS should implement fallocate regardless.
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