This series of patches does four things:
(1) Adds a generic intermediary (FS-Cache) by which filesystems may call on
local caching capabilities, and by which local caching backends may make
caches available:
+---------+
| | +-----------+
| NFS |--+ | |
| | | +-->| CacheFS |
+---------+ | +----------+ | | /dev/hda5 |
| | | | +-----------+
+---------+ +-->| | |
| | | |--+ +-------------+
| AFS |----->| FS-Cache | | |
| | | |----->| Cache Files |
+---------+ +-->| | | /var/cache |
| | |--+ +-------------+
+---------+ | +----------+ |
| | | | +-------------+
| ISOFS |--+ | | |
| | +-->| ReiserCache |
+---------+ | / |
+-------------+
(2) Adds a quasi-filesystem (CacheFS) that can turn block devices into a
local caches.
(3) Modifies the kAFS network filesystem to be able to read through this
cache.
(4) Documents the netfs interface and the cache backend interface.
Other backends may be added to the system, and other netfs's may be modified
to use caching.
There are a number of reasons why I'm not using i_mapping to do this. These
have been discussed a lot on the LKML and CacheFS mailing lists, but to
summarise the basics:
(1) Most filesystems don't do hole reportage. Holes in files are treated as
blocks of zeros and can't be distinguished otherwise, making it difficult
to distinguish blocks that have been read from the network and cached from
those that haven't.
(2) The backing inode must be fully populated before being exposed
to userspace through the main inode because the VM/VFS goes directly to
the backing inode and does not interrogate the front inode on VM ops.
Therefore:
(a) The backing inode must fit entirely within the cache.
(b) All backed files currently open must fit entirely within the cache at
the same time.
(c) A working set of files in total larger than the cache may not be
cached.
(d) A file may not grow larger than the available space in the cache.
(e) A file that's open and cached, and remotely grows larger than the
cache is potentially stuffed.
(3) Writes go to the backing filesystem, and can only be transferred to the
network when the file is closed.
(4) There's no record of what changes have been made, so the whole file must
be written back.
(5) The pages belong to the backing filesystem, and all metadata associated
with that page are relevant only to the backing filesystem, and not
anything stacked atop it.
David
