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

[Esi Y]

First of all, thanks for spotting this!

I believe you will still be limited to 128K, this could be tuned up to
max 1M [1] (current file limit size in pmxcfs, coincidentally), but
needs FUSE 3.6.0 [2]:

[1] https://github.com/torvalds/linux/commit/5da784cce4308ae10a79e3c8c41b13fb9568e4e0
[2] https://github.com/libfuse/libfuse/blob/master/ChangeLog.rst#libfuse-360-2019-06-13

Obviously, this should be buffered within the pmxcfs logic instead, however.

-- 
Esi Y

On Thu, Sep 19, 2024 at 11:52 AM Dominik Csapak <d.csapak at proxmox.com> wrote:
>
> 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.
>
> If we'd change to libfuse3, this would be a non-issue, since that option
> got removed and is the default there.
>
> Signed-off-by: Dominik Csapak <d.csapak at proxmox.com>
> ---
> sending as RFC, since I'm not sure if there are maybe any sideeffects i
> overlooked. I scanned the pmxcfs code for potential problems where we
> would e.g. assume a maximum size of 4096 for fuse writes, but could not
> find any. (also my C foo is not that great). I run it here currently,
> and it seems to work just fine atm.
>
>  src/pmxcfs/pmxcfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/src/pmxcfs/pmxcfs.c b/src/pmxcfs/pmxcfs.c
> index 1cf8ab8..c5b8f04 100644
> --- a/src/pmxcfs/pmxcfs.c
> +++ b/src/pmxcfs/pmxcfs.c
> @@ -945,7 +945,7 @@ int main(int argc, char *argv[])
>
>         mkdir(CFSDIR, 0755);
>
> -       char *fa[] = { "-f", "-odefault_permissions", "-oallow_other", NULL};
> +       char *fa[] = { "-f", "-odefault_permissions", "-oallow_other", "-obig_writes", NULL};
>
>         struct fuse_args fuse_args = FUSE_ARGS_INIT(sizeof (fa)/sizeof(gpointer) - 1, fa);
>
> --
> 2.39.2
>
>
>
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