FUD about CGD and GBDE

[Brian Fundakowski Feldman]

On Sat, Mar 05, 2005 at 05:37:47AM +1100, Peter Jeremy wrote:
> [CC list pruned]
> 
> On Wed, 2005-Mar-02 13:15:49 -0800, ALeine wrote:
> >If only hardware manufacturers were to equip hard drives with
> >a mechanism to ensure atomic writes. A capacitor large enough
> >to hold enough energy to flush the cache upon detecting the
> >power supply was cut would be sufficient.
> 
> I'm not sure thus is readily practical at the drive level.  Based
> on some back-of-envelope calculations using figures in a Seagate
> Barracuda manual (which I happened to have handy):
> - Random seek is 9.5 msec.
> - Rotational period is 8.3msec
> - Power consumption at 50% R/W, 50% seek is 0.82A @ 12V + 0.68A @ 5V
> 
> A single random seek + track write will take 17.8 msec.  This translates
> to an electrical charge of 0.015C @ 12V and 0.012C @ 5V.
> 
> Assuming the drive is designed to allow the supply rails to droop 20%
> whilst functioning correctly during this shutdown phase (which is a
> significantly bigger drop than the standard specifications), a single
> random seek + track write would require the drive to include a 6000µF
> capacitor on the 12V rail and a 12000µF capacitor on the 5V rail.
> 
> As a first order approximation all Unix disk operations are writes
> (reads can be satisfied from the buffer cache).  Given the size of
> current generation drive caches, it's more likely that there are
> around 50 writes cached - which requires capacitors 50 times as large
> - which would make them significantly larger than the drive itself.

This is not what anyone is asking to achieve.  What is being asked is
not to lose a whole track just because one sector on the track was
being written upon power loss.  Dealing with many in-order writes
being lost (in order), upon power loss, is something the filesystems
are already written to assume, and the fsync(2) semantics are designed
to work with.

-- 
Brian Fundakowski Feldman                           \'[ FreeBSD ]''''''''''\
  <> green at FreeBSD.org                               \  The Power to Serve! \
 Opinions expressed are my own.                       \,,,,,,,,,,,,,,,,,,,,,,\