[pve-devel] [RFC PATCH pve-cluster] fix #5728: pmxcfs: allow bigger writes than 4k for fuse

[Thomas Lamprecht]

Am 19/09/2024 um 14:45 schrieb Dominik Csapak:
> On 9/19/24 14:01, Thomas Lamprecht wrote:
>> Am 19/09/2024 um 11:52 schrieb Dominik Csapak:
>>> by default libfuse2 limits writes to 4k size, which means that on writes
>>> bigger than that, we do a whole write cycle for each 4k block that comes
>>> in. To avoid that, add the option 'big_writes' to allow writes bigger
>>> than 4k at once.
>>>
>>> This should improve pmxcfs performance for situations where we often
>>> write large files (e.g. big ha status) and maybe reduce writes to disk.
>>
>> Should? Something like before/after for benchmark numbers, flamegraphs
>> would be really good to have, without those it's rather hard to discuss
>> this, and I'd like to avoid having to do those, or check the inner workings
>> of the affected fuse userspace/kernel code paths here myself.
> 
> well I mean the code change is relatively small and the result is rather clear:

Well sure the code change is just setting an option... But the actual change is
abstracted away and would benefit from actually looking into..

> in the current case we have the following calls from pmxcfs (shortened for e-mail)
> when writing a single 128k block:
> (dd if=... of=/etc/pve/test bs=128k count=1)

Better than nothing but still no actual numbers (reduced time, reduced write amp
in combination with sqlite, ...), some basic analysis over file/write size distribution
on a single node and (e.g. three node) cluster, ...
If that's all obvious for you then great, but as already mentioned in the past, I
want actual data in commit messages for such stuff, and I cannot really see a downside
of having such numbers.

Again, as is I'm not really seeing what's to discuss, you send it as RFC after
all.

> [...] 
> so a factor of 32 less calls to cfs_fuse_write (including memdb_pwrite)

That can be huge or not so big at all, i.e. as mentioned above, it would we good to
measure the impact through some other metrics.

And FWIW, I used bpftrace to count [0] with an unpatched pmxcfs, there I get
the 32 calls to cfs_fuse_write only for a new file, overwriting the existing
file again with the same amount of data (128k) just causes a single call.
I tried using more data (e.g. from 128k initially to 256k or 512k) and it's
always the data divided by 128k (even if the first file has a different size)

We do not override existing files often, but rather write to a new file and
then rename, but still quite interesting and IMO really showing that just
because this is 1 +-1 line change it doesn't necessarily have to be trivial
and obvious in its effects.

[0]: bpftrace -e 'u:cfs_fuse_write /str(args->path) == "/test"/ {@ = count();} END { print(@) }' -p "$(pidof pmxcfs)"


>>> If we'd change to libfuse3, this would be a non-issue, since that option
>>> got removed and is the default there.
>>
>> I'd prefer that. At least if done with the future PVE 9.0, as I do not think
>> it's a good idea in the middle of a stable release cycle.
> 
> why not this change now, and the rewrite to libfuse3 later? that way we can
> have some improvements now too...

Because I want some actual data and reasoning first, even if it's quite likely
that this improves things Somehow™, I'd like to actually know in what metrics
and by how much (even if just an upper bound due to the benchmark or some
measurement being rather artificial).

I mean you name the big HA status, why not measure that for real? like, probably
among other things, in terms of bytes hitting the block layer, i.e. the actual
backing disk from those requests as then we'd know for real if this can reduce
the write load there, not just that it maybe should.