Re: [PATCH 1/6] libata: Do not retry commands with valid autosense

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2015-08-03 at 11:55 -0400, 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.

I'm not arguing that *this* patch is the best way to do it.  You asked
*why* autosense and that's what I answered.  I think there's time to
work out the implementation details to get them to be correct and well
structured.

James