Re: Roadmap for netfslib and local caching (cachefiles)

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi David,

On 2024/1/25 22:02, David Howells wrote:
> Here's a roadmap for the future development of netfslib and local caching
> (e.g. cachefiles).

Thanks for writing this detailed email.  And congrats to you work.
I only comment the parts directly related to myself.

> 

...

> 
> 
> Local Caching
> =============
> 
> There are a number of things I want to look at with local caching:
> 
> [>] Although cachefiles has switched from using bmap to using SEEK_HOLE and
> SEEK_DATA, this isn't sufficient as we cannot rely on the backing filesystem
> optimising things and introducing both false positives and false negatives.
> Cachefiles needs to track the presence/absence of data for itself.

Yes, that is indeed an issue that needs to resolve and already discussed
before.

> 
> I had a partially-implemented solution that stores a block bitmap in an xattr,
> but that only worked up to files of 1G in size (with bits representing 256K
> blocks in a 512-byte bitmap).

Jingbo once had an approach to use external bitmap files and
extended-attribute pointers inside the cache files:
https://listman.redhat.com/archives/linux-cachefs/2022-August/007050.html

I'm not quite sure the performance was but if it's worth trying or comparing,
that might be useful though.

> 
> [>] An alternative cache format might prove more fruitful.  Various AFS
> implementations use a 'tagged cache' format with an index file and a bunch of
> small files each of which contains a single block (typically 256K in OpenAFS).
> 
> This would offer some advantages over the current approach:
> 
>   - it can handle entry reuse within the index
>   - doesn't require an external culling process
>   - doesn't need to truncate/reallocate when invalidating
> 
> There are some downsides, including:
> 
>   - each block is in a separate file

Not quite sure, yet accessing too many small files might be another issue
which is currently happening with AI training workloads.. but as you said,
it's worth trying.

>   - metadata coherency is more tricky - a powercut may require a cache wipe
>   - the index key is highly variable in size if used for multiple filesystems
> 
> But OpenAFS has been using this for something like 30 years, so it's probably
> worth a try.

Yes, also configurable chunk sizes per blob are much helpful.

Thanks,
Gao Xiang

> 
> [>] Need to work out some way to store xattrs, directory entries and inode
> metadata efficiently.
> 
> [>] Using NVRAM as the cache rather than spinning rust.
> 
> [>] Support for disconnected operation to pin desirable data and keep
> track of changes.
> 
> [>] A user API by which the cache for specific files or volumes can be
> flushed.
> 
> 
> Disconnected Operation
> ======================
> 
> I'm working towards providing support for disconnected operation, so that,
> provided you've got your working set pinned in the cache, you can continue to
> work on your network-provided files when the network goes away and resync the
> changes later.
> 
> This is going to require a number of things:
> 
>   (1) A user API by which files can be preloaded into the cache and pinned.
> 
>   (2) The ability to track changes in the cache.
> 
>   (3) A way to synchronise changes on reconnection.
> 
>   (4) A way to communicate to the user when there's a conflict with a third
>       party change on reconnect.  This might involve communicating via systemd
>       to the desktop environment to ask the user to indicate how they'd like
>       conflicts recolved.
> 
>   (5) A way to prompt the user to re-enter their authentication/crypto keys.
> 
>   (6) A way to ask the user how to handle a process that wants to access data
>       we don't have (error/wait) - and how to handle the DE getting stuck in
>       this fashion.
> 
> David