Re: [PATCH 4/5] fuse: writeback_cache consistency enhancement (writeback_cache_v2)

[Miklos Szeredi]

On Tue, 11 Jul 2023 at 06:36, Jiachen Zhang
<zhangjiachen.jaycee@xxxxxxxxxxxxx> wrote:
>
> Some users may want both the high performance of the writeback_cahe mode
> and a little bit more consistency among FUSE mounts. Current
> writeback_cache mode never updates attributes from server, so can never
> see the file attributes changed by other FUSE mounts, which means
> 'zero-consisteny'.
>
> This commit introduces writeback_cache_v2 mode, which allows the attributes
> to be updated from server to kernel when the inode is clean and no
> writeback is in-progressing. FUSE daemons can select this mode by the
> FUSE_WRITEBACK_CACHE_V2 init flag.
>
> In writeback_cache_v2 mode, the server generates official attributes.
> Therefore,
>
>     1. For the cmtime, the cmtime generated by kernel are just temporary
>     values that are never flushed to server by fuse_write_inode(), and they
>     could be eventually updated by the official server cmtime. The
>     mtime-based revalidation of the fc->auto_inval_data mode is also
>     skipped, as the kernel-generated temporary cmtime are likely not equal
>     to the offical server cmtime.
>
>     2. For the file size, we expect server updates its file size on
>     FUSE_WRITEs. So we increase fi->attr_version in fuse_writepage_end() to
>     check the staleness of the returning file size.
>
> Together with FOPEN_INVAL_ATTR, a FUSE daemon is able to implement
> close-to-open (CTO) consistency like NFS client implementations.

What I'd prefer is mode similar to NFS: getattr flushes pending writes
so that server ctime/mtime are always in sync with client.  FUSE
probably should have done that from the beginning, but at that time I
wasn't aware of the NFS solution.

Thanks,
Miklos