On 09/25/2012 04:08 PM, James Bottomley wrote:
On Tue, 2012-09-25 at 01:21 -0400, Jeff Garzik wrote:
On 09/25/2012 12:06 AM, James Bottomley wrote:
On Mon, 2012-09-24 at 17:00 -0400, Jeff Garzik wrote:
drivers/scsi/sd.c | 4 ++++
drivers/scsi/sd.h | 2 +-
2 files changed, 5 insertions(+), 1 deletion(-)
I'm not opposed in principle to doing this (except that it should be a
sysfs parameter like all our other controls), but what's the reasoning
behind needing it changed?
<vendor hat on>
Periodically turns up as a useful field sledgehammer for solving
problems, until the real problem is found and fixed. Got tired of a
very similar patch manually bouncing around the "hey, pssst, this worked
for me" backchannel IT network.
</red hat>
I'm asking because the general consensus from the device guys is that we
should never retry unless the device or the transport tells us to (and
then we shouldn't count the retries). A long time ago we used to get
spurious command failures from retry exhaustion on QUEUE_FULL or BUSY,
but since we switched those to being purely timeout based, I thought the
problem had gone away and I'm curious to know what guise it resurfaced
in.
I think that is still very much a true statement. By the time normal disks
return an error, they have retried *many* times in firmware. There are some
exceptions of course - vibrations and so on might make this useful.
Back when my day job often involved recovering data from dead drives, we
actually normally wanted to cut retries down to zero since various part of the
stack retried for us so much that each bad sector had to be timed out multiple
times!
I don't object to making this a tunable, but we should default to not retrying.
Also would be very interesting to seeing if this actually is useful in the real
world, not just "word on the street" world :)
Ric
Can you be more specific about sysfs location? A runtime-writable (via
sysfs!) module parameter for a module-wide default seemed appropriate.
Well, if it's really important, the same thing should happen with
retries as happened with timeout (it became a request_queue property),
but it could be hacked as a struct scsi_disk one with a corresponding
entry in sd_dis_attrs.
James
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