On Feb 20, 2015, at 1:50 AM, Michael Kerrisk [off-list ref] wrote:

Hello Ted,

Based on your commit message 0ae45f63d4e, I I wrote the documentation
below for MS_LAZYTIME, to go into the mount(2) man page. Could you
please check it over and let me know if it's accurate. In particular,
I added pieces marked with "*" below that were not part of the commit
message and I'd like confirmation that they're accurate.

Thanks,

Michael

[[
      MS_LAZYTIME (since Linux 3.20)
             Only  update  filetimes (atime, mtime, ctime) on the in-
             memory version of the file  inode.   The  on-disk  time‐
             stamps are updated only when:

             (a)  the inode needs to be updated for some change unre‐
                  lated to file timestamps;

             (b)  the application  employs  fsync(2),  syncfs(2),  or
                  sync(2);

             (c)  an undeleted inode is evicted from memory; or

*             (d)  more than 24 hours have passed since the i-node was
*                  written to disk.

             This mount option significantly reduces  writes  to  the
             inode  table  for workloads that perform frequent random
             writes to preallocated files.

*             As at Linux 3.20, this option is supported only on ext4.

I _think_ that the lazytime mount option is generic for all filesystems.
I believe ext4 has an extra optimization for it, but that's it.

Cheers, Andreas