Canvassing for network filesystem write size vs page size

David Howells
2021-08-05T16:35:59Z

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.
- Max RPC size is therefore object size.
- Doesn't track bytes.

CIFS/SMB
- Writes back just changed bytes immediately under some circumstances
- 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.

Thanks,
David