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

[Thomas Lamprecht]

Am 14/10/2024 um 12:09 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 (namely up to 128 KiB).
> 
> This means that if we update a file with more than 4KiB data, the
> following pattern occurs:
> 
> * cfs_fuse_write is called with at offset 0 with 4096 size
> * sqlite writes the partial file to disk since it's a transaction
> * cfs_fuse_write is called with an offset 4096 and with 4096 size
> * sqlite updates the data and writes again
> * repeat until all data reached cfs_fuse_write
> 
> So when cfs_fuse_write accepts bigger chunks, we have less
> cfs_fuse_write -> sqlite write cycles, leading to a reduced disk
> activity.
> 
> Note that sqlite itself uses 4096 byte blocks to write to the file
> system layer below.
> 
> Most files on pmxcfs are written with `file_set_contents`, which writes
> the file into a tmp file and renames it, so we always have some write
> overhead.
> 
> Previous to pve-common commit
> ef0bcc9 (tools: file_set_contents: use syswrite instead of print)
> 
> it used `print` to write, which uses an internal 8k buffer, and after
> the commit it uses `syswrite`, which writes the file unbuffered in one
> go. (Fuse still splits writes at it's defined maximum)
> 
> The commit message of that patch includes benchmarks for various sizes
> of writes on pmxcfs with this patch included. Results show that we can
> reduce the amount of bytes written to disk for files larger than 4 KiB
> by a significant amount (with both patches we can reduce the
> amplification at 8KiB from ~15x to ~11x, and for 1024KiB from ~360x to
> ~15x)
> 
> When we change to libfuse3, we have to remove this option again, since
> it got removed and is the default there.
> 
> Signed-off-by: Dominik Csapak <d.csapak at proxmox.com>
> ---
> changes from rfc:
> * improve commit message to contain more detail and reference
>   Filips commit that improves `file_set_contents`
> * add a comment above the option to remove it with change to libfuse3
> 
>  src/pmxcfs/pmxcfs.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
>

applied, many thanks for the elaborate commit message!