Adding Bob Sheffield from Broadcom. > > Hi Kashyap, > > > I was going through below discussion as well as going through linux > > scsi code to know if linux scsi stack support NDOB. > > Last time NDOB came up there were absolutely no benefits to it from the > kernel perspective. These days we can save the buffer memory allocation so > there may be a small win. I do have a patch we can revive. We can test if you have any patch. > > However, I am not aware of any devices that actually support NDOB. Plus it's > hard to detect since we need to resort to RSOC masks. And blindly sending > RSOC is risky business. That's why my patch never went anywhere. It was a lot > of heuristics churn to set a single bit flag. > > Since the benefits are modest (basically saves a memory compare on the > device), what is the reason you are looking at this? SCSI SBC-4 requires that any drive that supports WRITE SAME (unmap=1) also support ndob=1. So a drive supports it if it reports LBWS=1 in the Block Provisioning VPD page. If not, the drive violates the SBC-4 standard, so it's a drive problem. ssuing WRITE SAME (unmap=1, ndob=0) only achieves block unmapping if the pattern in the data-out buffer matches the "provisioning initialization pattern" implemented by the drive. AFAIK, current scsi stack isn't somehow determining what that pattern is for each drive, so eventually current method of using WRITE SAME (unmap=1, ndob=0) likely is ineffective in unmapping blocks on media. On the other hand, if the drive supports WRITE SAME (unmap=1, ndob=1) - as required by SBC-4 - then Linux can use it to reliably cause LBAs to be unmapped on media. Perhaps this is considered a minor issue for direct attached drives, but in the RAID world, it's a big enough issue that relying on it is the only way we can reliably maintain coherency across stripes in redundant data mappings. > > > One more question. What happens if WS w/ UNMAP command is passed to > > the device without zeroed data out buffer in current scsi stack ? Will > > it permanently disable WS on that device ? > > Depends how the device responds. > > -- > Martin K. Petersen Oracle Linux Engineering
