Re: Canvassing for network filesystem write size vs page size

[Jeff Layton <jlayton@redhat.com>]

On Thu, 2021-08-05 at 17:35 +0100, David Howells wrote:
> With Willy's upcoming folio changes, from a filesystem point of view, we're
> going to be looking at folios instead of pages, where:
> 
>  - a folio is a contiguous collection of pages;
> 
>  - each page in the folio might be standard PAGE_SIZE page (4K or 64K, say) or
>    a huge pages (say 2M each);
> 
>  - a folio has one dirty flag and one writeback flag that applies to all
>    constituent pages;
> 
>  - a complete folio currently is limited to PMD_SIZE or order 8, but could
>    theoretically go up to about 2GiB before various integer fields have to be
>    modified (not to mention the memory allocator).
> 
> Willy is arguing that network filesystems should, except in certain very
> special situations (eg. O_SYNC), only write whole folios (limited to EOF).
> 
> Some network filesystems, however, currently keep track of which byte ranges
> are modified within a dirty page (AFS does; NFS seems to also) and only write
> out the modified data.
> 
> Also, there are limits to the maximum RPC payload sizes, so writing back large
> pages may necessitate multiple writes, possibly to multiple servers.
> 
> What I'm trying to do is collate each network filesystem's properties (I'm
> including FUSE in that).
> 
> So we have the following filesystems:
> 
>  Plan9
>  - Doesn't track bytes
>  - Only writes single pages
> 
>  AFS
>  - Max RPC payload theoretically ~5.5 TiB (OpenAFS), ~16EiB (Auristor/kAFS)
>  - kAFS (Linux kernel)
>    - Tracks bytes, only writes back what changed
>    - Writes from up to 65535 contiguous pages.
>  - OpenAFS/Auristor (UNIX/Linux)
>    - Deal with cache-sized blocks (configurable, but something from 8K to 2M),
>      reads and writes in these blocks
>  - OpenAFS/Auristor (Windows)
>    - Track bytes, write back only what changed
> 
>  Ceph
>  - File divided into objects (typically 2MiB in size), which may be scattered
>    over multiple servers.

The default is 4M in modern cephfs clusters, but the rest is correct.

>  - Max RPC size is therefore object size.
>  - Doesn't track bytes.
> 
>  CIFS/SMB
>  - Writes back just changed bytes immediately under some circumstances

cifs.ko can also just do writes to specific byte ranges synchronously
when it doesn't have the ability to use the cache (i.e. no oplock or
lease). CephFS also does this when it doesn't have the necessary
capabilities (aka caps) to use the pagecache.

If we want to add infrastructure for netfs writeback, then it would be
nice to consider similar infrastructure to handle those cases as well.

>  - Doesn't track bytes and writes back whole pages otherwise.
>  - SMB3 has a max RPC size of 16MiB, with a default of 4MiB
> 
>  FUSE
>  - Doesn't track bytes.
>  - Max 'RPC' size of 256 pages (I think).
> 
>  NFS
>  - Tracks modified bytes within a page.
>  - Max RPC size of 1MiB.
>  - Files may be constructed of objects scattered over different servers.
> 
>  OrangeFS
>  - Doesn't track bytes.
>  - Multipage writes possible.
> 
> If you could help me fill in the gaps, that would be great.


-- 
Jeff Layton <jlayton@redhat.com>