Re: Problem identified: WAPL/RAIDframe performance problems

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Tue, Dec 04, 2012 at 02:14:27PM +0000, David Holland wrote:
> On Sat, Dec 01, 2012 at 11:38:55PM -0500, Mouse wrote:
>  > >>> things.  What I care about is the largest size "sector" that will (in
>  > >                                     ^^^^^^^
>  > >>> the ordinary course of things anyway) be written atomically.
>  > >> Then those are 512-byte-sector drives [...]
>  > > No; because I can do 4K atomic writes, I want to know about that.
>  > 
>  > And, can't you do that with traditional drives, drives which really do
>  > have 512-byte sectors?  Do a 4K transfer and you write 8 physical
>  > sectors with no opportunity for any other operation to see the write
>  > partially done.  Is that wrong, or am I missing something else?
> 
> Insert a kernel panic (or power failure(*)) after five sectors and

What's a kernel panic got to do with it?  If you hand the controller
and thus the drive 4K write, the kernel panicing won't suddenly cause
you to reverse time and have issued 8 512-byte writes instead.

Given how drives actually write data, I would not be so sanguine
that any sector, of whatever size, in-flight when the power fails,
is actually written with the values you expect, or not written
at all.