[linux-lvm] Re: [PATCH] 64 bit scsi read/write

[Jonathan Lundell <jlundell pobox com>]

At 9:45 AM +0100 2001-07-14, Alan Cox wrote:
>  > If, after a power outage, the IDE disk can keep going for long enough
> >  to write its write cache out to the reserved vendor area (which will
> >  only take 20-30 milliseconds) then the data may be considered *safe*
> >  as soon as it hits writecache.
>
> Hohohoho.
>
> >  In which case it is perfectly legitimate and sensible for the drive
> >  to ignore flush commands, and to ack data as soon as it hits cache.
>
> Since the flushing commands are 'optional' it can legitimately ignore them
>
> >  If I'm right then the only open question is: which disks do and
> >  do not do the right thing when the lights go out.
>
> As far as I can tell none of them at least in the IDE world

It's not so great in the SCSI world either. Here's a bit from the 
Ultrastar 73LZX functional spec (this is the current-technology 
Ultra160 73GB family):

> 5.0 Data integrity
> The drive retains recorded information under all non-write operations.
> No more than one sector will be lost by power down during write 
> operation while write cache is
> disabled.
> If power down occurs before completion of data transfer from write 
> cache to disk while write cache is
> enabled, the data remaining in write cache will be lost. To prevent 
> this data loss at power off, the
> following action is recommended:
> * Confirm successful completion of SYNCHRONIZE CACHE (35h) command.

What's worse, though the spec is not explicit on this point, it 
appears that the write cache is lost on a SCSI reset, which is 
typically used by drivers for last-resort error recovery. And of 
course a SCSI bus reset affects all the drives on the bus, not just 
the offending one.
--
/Jonathan Lundell.