Re: Exposing FUA as alternative to DIOCCACHESYNC for WAPBL

[mlelstv%serpens.de@localhost (Michael van Elst)]

tls%panix.com@localhost (Thor Lancelot Simon) writes:

>It's not obvious, but in fact ORDERED gets set for writes
>as a default, I believe -- in sd.c, I think?

>This confused me for some time when I last looked at it.


sd always asks for simple tags when issuing read/write commands
and head tags when issuing a START_STOP command.

Any other commands are left to scsipi_base which sets head tags
for XS_CTL_URGENT and simple tags otherwise.

The only urgent command from sd is again START_STOP (but which
already asks for a head tag).

The other urgent command is a REQUEST_SENSE.



>Setting WCE on SCSI drives is simply a bad idea.  It is
>not necessary for performance and creates data integrity
>isues.

I don't know details about data integrity issues although
I'm sure there are some. But unfortunately WCE makes a difference
on many SCSI drives nowadays. You either run with WCE or
use a RAID controller with it's own stable storage (BBU/Flash)
or live with a significant speed penalty for writes.


>I don't believe most SCSI drives are run with WCE on.

In my experience about 20-30% of SAS drives default to WCE on
(and >90% of SATA drives).

-- 
-- 
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."