>>>>> "Mikulas" == Mikulas Patocka <mpatocka@xxxxxxxxxx> writes: Mikulas> Propagating the limits up the tree won't work anyway because of Mikulas> a race condition. Think of this case: Mikulas> 1. We test queue limits and find out that the device supports Mikulas> WRITE_SAME Mikulas> 2. We build a WRITE_SAME bio to be submitted. Mikulas> 3. The user reconfigures a device so that it no longer supports Mikulas> WRITE_SAME Mikulas> (for example, he adds a snapshot or converts linear volume to a Mikulas> mirror). Mikulas> 4. We submit the bio that we built in step 2. Mikulas> 5. The bio fails. The existing "fall back to writes if write same fails" mechanism isn't going away. We absolutely need that. The principal headache is caused by host adapters that perform SCSI-ATA or SCSI-<internal RAID implementation> translation and which do not correctly fail a WRITE SAME command that we send down to the disk. Based on heuristics from querying the target we expect WRITE SAME to succeed so we enable it. But for whatever reason the controller is not able to handle the command. Most controllers/drivers will fail the command correctly and we'll disable WRITE SAME and fall back to manually zeroing. Some controllers, however, hang or fail the I/O in a way in which we do not pick up it's a WRITE SAME problem. The other headache is what happens if a stacking driver gets -EIO or -EREMOTEIO on a WRITE SAME request. That's what Mike was trying to fix with his patch. Maybe it would make sense for us to use -EINVAL or something similar to single out the ILLEGAL REQUEST case. While -EREMOTEIO specifically calls out for "no retry" it might be helpful to indicate that we're dealing with an unsupported command. That way DM or MD can reconfigure themselves accordingly. -- Martin K. Petersen Oracle Linux Engineering -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
