t...@panix.com (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: mlel...@serpens.de "A potential Snark may lurk in every tree."