>>>>> "Mike" == Mike Snitzer <snitzer@xxxxxxxxxx> writes: Mike, Mike> AFAIK the reason for these heuristics is: devices that do support Mike> WRITE SAME cannot properly report as much because they don't Mike> support REPORT SUPPORTED OPERATION CODES -- this lack of RSOC Mike> support is apparently very common? Only a handful of the very latest and greatest devices support RSOC. The number of devices that support WRITE SAME is orders of magnitude larger. Last I checked I had exactly 1 out of about 100 devices in my lab that supported RSOC. Mike> I can appreciate the idea behind the current heuristics but I Mike> think the prevelence of the other side of the spectrum (SCSI Mike> devices that don't support RSOC or WRITE SAME) was underestimated. If you by "devices" mean vintage PCI RAID controllers that don't pass things through correctly, then yes. I don't think I have a single SCSI drive that doesn't support WRITE SAME. And all the controllers I tested with here worked fine. Mike> As I say in that comment: "A proper fix could be to make SCSI's Mike> default be to disable WRITE SAME for devices that don't properly Mike> report they support it. And possibly have a whitelist to opt-in Mike> to enabling WRITE SAME for select targets." The problem with the opt-in approach is that there are orders of magnitude more devices that would need to get it enabled than there are broken ones that need it disabled. There are only a couple of handfuls of RAID controller drivers. We've been working through the issues on these on a case by case basis. Yes, I totally agree it sucks. And I hate that things broke for people with Areca and 3ware. But we got those fixed. And it's way easier to blacklist "all devices hanging off RAID driver xyz" than it is to whitelist every SCSI drive known to man. It sucks in the short term but is better long term. The major headache here of course is that WRITE SAME is inherently destructive. We can't just fire off one during discovery and see if it works. For WRITE you can issue a command with a transfer length of 0 to see if things work. But unfortunately for WRITE SAME a transfer length of zero means "wipe the entire device". Yikes! I guess we could read one sector and try to write it back using WRITE SAME and a block count of one. But it's really icky. And I don't like the notion of actually writing things during discovery. As far as being able to trigger a restacking of the queue limits I think it's inevitable. We see more and more devices that change properties after a firmware upgrade. I think we'll just have to bite the bullet and work on that... -- Martin K. Petersen Oracle Linux Engineering -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
