USB storage devices that support SAT (the T10 SCSI to ATA translation standard) are beginning to appear. SAT enables tools like smartmontools to access SMART data on a ATA disk in a USB enclosure. We have run into a problem. It seems that the usb storage subsystem is wedded to the idea of sense data that is no longer than 18 bytes. ** That doesn't play well with SAT which uses descriptor format sense data that is made up of an 8 byte header plus zero or more descriptors. SAT uses a 14 byte "ATA return" (sense) descriptor to yield the ATA registers. That means the sense data length is 22 bytes when an ATA return descriptor is required. Alan Stern has already noted to another smartmontools developer that such a change is likely to break some USB storage devices. Perhaps the maximum sense buffer size could be optionally specified per usb storage device. Alternatively the usb mass storage logic could make some dynamic decisions itself. For example if the (disk) device responds successfully to either SCSI ATA PASS_THROUGH (12 or 16 byte) command then it will be capable of (at least) 22 byte sense data. A T10 vendor identification field in a standard INQUIRY response of "ATA " is another indication of a device that supports SAT. ** the 18 byte sense data limit comes from SCSI-2 (circa 1992). That was upped to 252 bytes around 10 years ago. Some SCSI disk vendors have been using the extra bytes (above 18) for some time. Then in SPC-3 (standard in 2005) descriptor format sense data was introduced and SAT uses it. There are other uses: any SCSI (virtual) disk that needs more than 32 bits to represent all its LBAs should use descriptor format for MEDIUM ERRORs. The reason is that MEDIUM ERRORs should include the LBA of the first failure and the "info" field in the older "fixed" sense format is only 4 bytes. Doug Gilbert -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
