On 08/03/2015 05:55 PM, Tejun Heo wrote: > Hello, James. > > On Mon, Aug 03, 2015 at 08:42:43AM -0700, James Bottomley wrote: >> I'd think it would be the same reason as all modern transports: it's >> faster and allows processing of sense data in-band. Under the old >> regime, the device is effectively frozen until you collect the data. >> Under autosense, the data is collected as part of the in-band command >> processing, so it doesn't stall the device. >> >> Modern drives (and protocols) are moving towards being somewhat more >> chatty with sense data. It doesn't just signal an error, mostly it's >> just reporting about drive characteristics or other advisory stuff. >> This means that if you handle it the old way, you'll get more drive >> stalls and a corresponding reduction in throughput. > > The problem is not the "auto" part but the "sense" part, I guess. ATA > devices (the harddisks) never reported sense data and instead had a > more rudimentary error bits and for newer devices NCQ log pages, so > libata EH decodes those error information and takes appropriate > actions for the indicated error condition. > > Hannes's patchset makes ATA devices mostly bypass libata EH when sense > data is present. For, say, unrecoverable read errors, it'd be > possible to make this scheme work (broken currently tho); however, > libata and SCSI aren't that closely tied and there currently is no way > for SCSI to tell libata that, e.g., link error was detected on the > device side, so libata will fail to take link recovery actions on > those cases. > > This *can* be made to work in a couple different ways but what's > implemented now is pretty broken and making it work properly in any > other way than integrating sense decoding into libata EH would require > major restructuring of the whole thing which I'm not sure would be > worthwhile at this point. > At the moment NCQ autosense is mostly used to provide the host with more details for a failed I/O. The typical case here is (no small surprise) ZAC disks, which use autosense to inform the host about a malformed I/O. It is _not_ being used as a replacement for existing error behaviour, (ie link errors are not being signalled with that; how could they if there is no link?); in fact, during testing I"ve seen both, autosense I/O failures and normal I/O failures for which autosense is not set, and the normal error handling kicks in. It's not that I've disable the original error handler completely, it's only bypassed for I/O failure where a sense code is provided. And the drive surely knows which error occurs, so we'd be daft not be using that. So I think disabling autosense completely is a bit extreme... Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@xxxxxxx +49 911 74053 688 SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg) -- 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
