mount.2: Document MS_LAZYTIME

Based on Ted T'so's commit message 0ae45f63d4e

Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cowritten-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/mount.2

@@ -151,6 +151,32 @@ Make directory changes on this filesystem synchronous.
 or subtrees using
 .BR chattr (1).)
 .TP
+.BR MS_LAZYTIME " (since Linux 3.20)"
+.\" commit 0ae45f63d4ef8d8eeec49c7d8b44a1775fff13e8
+.\" commit fe032c422c5ba562ba9c2d316f55e258e03259c6
+.\" commit a26f49926da938f47561f386be56a83dd37a496d
+Reduce on-disk updates of inode timestamps (atime, mtime, ctime)
+by maintaining these changes only in memory.
+The on-disk timestamps are updated only when:
+.RS
+.IP (a) 5
+the inode needs to be updated for some change unrelated to file timestamps;
+.IP (b)
+the application employs
+.BR fsync (2),
+.BR syncfs (2),
+or
+.BR sync (2);
+.IP (c)
+an undeleted inode is evicted from memory; or
+.IP (d)
+more than 24 hours have passed since the inode was written to disk.
+.RE
+.IP
+This mount option significantly reduces writes to the inode table
+for some workloads
+(e.g., when performing frequent random writes to preallocated files).
+.TP
 .B MS_MANDLOCK
 Permit mandatory locking on files in this filesystem.
 (Mandatory locking must still be enabled on a per-file basis,