Re: [PATCH 0/5 v2] add extent status tree caching

[Theodore Ts'o <tytso@mit.edu>]

On Tue, Jul 30, 2013 at 01:08:07PM +1000, Dave Chinner wrote:
> But Ted's case is not a "hint" - it's a direct command to fetch the
> extent map from disk. You can do that already with FIEMAP, so no new
> code or interfaces are needed. fadvise() is not the proper interface
> for manipulating filesystem metadata behaviour, and fiemap can
> already do what you need. There is no need for any new interfaces
> here.

I've been looking at the definition of fiemap, and I'm not convinced.
To quote from the fiemap.txt:

   The fiemap ioctl is an efficient method for userspace to get file
   extent mappings.

That's not what is going on here.  We are pre-caching them into kernel
memory, not in user-space.  In addition, we're also setting a flag to
keep these extents preferentially in memory compared to other entries
in the extent cache.

I agree that posix_fadvise() isn't really a good match, either:

   "posix_fadvise - predeclare an access pattern for file data"

How about this?  FIEMAP is an ioctl, anyway.  How about if we just
declare this as a new fs-independent ioctl, much like FS_IOC_FIEMAP?

#define FS_IOC_PRECACHE_EXTENTS    _IO('f', 18)

This is, of course, assuming that other file systems are interested in
implementing this functionality.  If not, we can just keep it as
EXT4_IOC_PRECACHE_EXTENTS, and just call it a day.  (We can always add
a definition of FS_IOC_PRECACHE_EXTENTS set to ext4 ioctl's code
point, at some later point, if people change their minds.)

			       	    	      - Ted