T10 WCE interpretation in Linux & device level access

[Ric Wheeler <rwheeler@redhat.com>]

For many years, we have used WCE as an indication that a device has a volatile 
write cache (not just a write cache) and used this as a trigger to send down 
SYNCHRONIZE_CACHE commands as needed.

Some arrays with non-volatile cache seem to have WCE set and simply ignore the 
command.

Some arrays with non-volatile cache seem to not set WCE.

Others arrays with non-volatile cache - our problem arrays - set WCE and do 
something horrible and slow when sent the SYNCHRONIZE_CACHE commands.

Note that for file systems, you can override this behavior by mounting with our 
barriers disabled (mount -o nobarrier .....). There is currently no way do 
disable this for anything using the device directly, not through the file system.

Some applications run against block devices - not through a file system - and 
want not to slow to a crawl when they have an array in my problem set.

Giving them a hook to ignore WCE seems to be a hack, but one that would resolve 
issues with users who won't want to wait months (years?) for us to convince the 
array vendors.

Is this a hook worth doing?

Have we hashed this out in the T10 committee?

Regards,

Ric