Re: [PATCH 0/12] FS-Cache: Generic filesystem caching facility

[Andrew Morton <akpm@osdl.org>]

David Howells <dhowells@redhat.com> wrote:
>
> Andrew Morton <akpm@osdl.org> wrote:
> 
> > > What I'm trying to do is actually fairly simple in concept:
> > > 
> > >   (1) Have a metadata inode (imeta) that covers the block device.
> > > 
> > 
> > Can you remind me again why it requires a blockdev rather than a regular file?
> 
> That's the third time you've asked:-)

Maybe on the fourth or fifth time it'll occur you to put it into the
changelog.

> ...
> Look also at Documentation/filesystem/caching/fscache.txt provided by patch
> 9/12 for the constraints I've set, in particular:
> 
>  (1) It must be practical to operate without a cache.
> 
>  (2) The size of any accessible file must not be limited to the size of the
>      cache.
> 
>  (3) The combined size of all opened files (this includes mapped libraries)
>      must not be limited to the size of the cache.
> 
>  (4) The user should not be forced to download an entire file just to do a
>      one-off access of a small portion of it (such as might be done with the
>      "file" program).
> 
> To which I wish to add:
> 
>  (5) The netfs pages must remain owned by the netfs, so that there is no
>      difference between the netfs operating with a cache and it operating
>      without a cache. This means I/O must be done to/from the netfs pages
>      directly from/to the cache.

None of that appears to be relevant.

A blockdev is just a big, fixed-sized file.  Why cannot it be backed by a
big, fixed-sized file?

<looks>

OK, it's doing submit_bio() directly.

> 
> I have a start of a cache-on-files facility (called, most imaginatively,
> CacheFiles) which works as another backend to FS-Cache. Of the underlying
> filesystem, it requires:
> 
>  (*) O_DIRECT
> 
>  (*) Reads and writes on arbitrary kernel pages
> 
>  (*) Reads on holes must return short or ENODATA. This requires an extra
>      O_XXXX flag to be supplied when opening a file or the struct file or
>      inode to be flagged appropriately.
> 
>  (*) The ability to issue FS operations such as rename, open, setxattr, mkdir
>      from kernel space.
> 
> This facility isn't well advanced yet, and will initially only be available on
> EXT2/3. It will also require a userspace component to clean up dead nodes.

I'd have thought that a decent intermediate step would be
cache-on-single-file using a_ops.direct_IO, as you're implying above.  Then
all the direct-to-blockdev code can go away.  It'll take some tweaking of
the core direct-io code, but nothing terribly serious.

> 
> Are you willing to at least carry the FS-Cache core and the AFS usage of it?
>

fs-cache won't do anything without a backing store such as cachefs will it?

Those names are rather confusing, btw.