Re: [Lsf-pc] [LSF/MM TOPIC] atomic block device

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

In addition to Dave's comments, consider the following from Val's
article with Alex quoted:

   The overall performance result was that the Featherstitch
   implementations were at par or somewhat better with the comparable
   ext3 version for elapsed time, but used significantly more CPU
   time.....

   So, you can use Featherstitch to re-implement all kinds of file
   system consistency schemes - soft updates, copy-on-write,
   journaling of all flavors - and it will go about as fast the old
   version while using up more of your CPU.

And note that this was comparing against ext3, which is not exactly a
shining example of performance.  (i.e., ext4 and xfs tend to beat ext3
handily on most benchmarks.)

Furthermore, given the sort of dependency tracking which Featherstich
is attempting, I suspect that the results will be at the very least
interesting on a system with a large number of cores; it's very likely
that it's CPU scalability leaves much to be desired.

Finally, note that many disk drives do not perform all that well with
writeback caching disabled (which is required for soft update and its
variants).  So when people do benchmarks comparing soft updates versus
traditional file systems, and important question to ask is (1) did
they remember to disable writeback caching for the soft updates run
(which is not the default, and if you don't disable it, you lose your
powerfail relibility), and (2) was writeback caching enabled or
disabled when benchmarking the traditional system, which can safely
use the default HDD writeback caching.

Regards,

					- Ted