More Adaptec 29320 + Seagate ST336607LW woes

[Don Lewis]

On  6 Dec, ian j hart wrote:
> On Friday 05 December 2003 8:33 pm, Don Lewis wrote:

>> I've been meaning to suggest that if you haven't done so already, use
>> "camcontrol modepage" to set the WCE bit to 0 which will disable write
>> caching on the drive.  In the testing that I've done, I haven't seen any
>> significant performance penalty in doing this if tagged command queuing
>> and softupdates are in use.  It is also safer for soft updates because
>> the drive no longer lies about when write data reaches stable storage.
> 
> I thought tagged queuing needed WCE.

Nope.  As near as I can tell, the drive always returns the "Done"
response to a write command immediately if WCE is on, and waits until
the data hits the platter if WCE is off.  The difference with tagged
queuing is that while the drive will attempt to do the write as soon as
possible, it will accept more than one command at a time and will
attempt to schedule the physical I/O in some sort of optimim order to
maximize throughput.  If the OS wants to enforce a particular order, it
can just wait for commands to complete before sending new ones to the
drive, which is pretty much what soft updates does.  I think there are
also some other ways for the OS to constrain the physical I/O ordering.

One place where folks have noticed a performance difference between WCE
on and WCE off is when using newfs after we got rid of the buffered
block devices and forced newfs to use the raw character device.  Newfs
only issues one write() at a time and waits for the status before
issuing another write().  If WCE is on, the writes only have to hit the
drive cache before the drive returns a status that gets returned to
userland, but if WCE is off the data has to hit the platter before the
status is returned, which slows down newfs quite considerably.  Tagged
queuing doesn't make any difference in this case because the drive will
only see at most only one command at a time.