New nanosecond stat patch for 2.5.44

Quoted 10 lines · Andi Kleen

Move time_t members in struct stat to struct timespec and allow subsecond timestamps for files. Too big to post on the list, because it edits a lot of file systems and drivers in a straight forward way.

This is required for reliable "make" on fast computers.

File systems that support nsec storage are currently: XFS, JFS, NFSv3 (if the filesystem on the server supports it), VFAT (not quite nanosecond), CIFS (unit in 100ns which is above what linux supports), SMBFS (for newer servers)

Two notes I might make about this:
1) It would be good if it were possible to select this with a config option (I don't care which way the default goes), so that people who don't need/care about the increased resolution don't need the extra space in their inodes and minor extra overhead. To make this a lot easier to code, having something akin to the inode_update_time() which does all of the i_[acm]time updates as appropriate.

2) Updating i_atime based on comparing the nsec timestamp is going to be a killer. I think AKPM saw dramatic performance improvements when he changed the code to only do the update once/second, and even though you are "only" updating the atime if the times are different, in practise this will be always. Even without the "per superblock interval" you suggest we should probably only update the atime once a second (I don't think anything is keyed off such high resolution atimes, unlike make and mtime/ctime).

3) The fields you are usurping in struct stat are actually there for the Y2038 problem (when time_t wraps). At least that's what Ted said when we were looking into nsec times for ext2/3. Granted, we may all be using 64-bit systems by 2038... I've always thought 64 bits is much to large for time_t, so we could always use 20 or 30 bits for sub-second times, and the remaining bits for extending time_t at the high end, and mask those off for now, but that is a separate issue...

Quoted 6 lines · H. Peter Anvin

We probably need to revamp struct stat anyway, to support a larger dev_t, and possibly a larger ino_t (we should account for 64-bit ino_t at least if we have to redesign the structure.) At that point I would really like to advocate for int64_t ts_sec and uint32_t ts_nsec and quite possibly a int32_t ts_taidelta to deal with leap seconds... I'd personally like struct timespec to look like the above everywhere.

For filesystems can we get away with just the 64-bit nanoseconds? By my calculations that gives something like 584 years--do we need to worry about files older than that?

Quoted 6 lines · Andreas Dilger

1) It would be good if it were possible to select this with a config option (I don't care which way the default goes), so that people who don't need/care about the increased resolution don't need the extra space in their inodes and minor extra overhead. To make this a lot easier to code, having something akin to the inode_update_time() which does all of the i_[acm]time updates as appropriate.

You're joking right? That's twelve bytes of more state per struct inode and I bet even with the most insidious micro benchmark you won't be able to detect a difference in speed from the basic manipulation.