On 12/13/2016 02:19 PM, Sumit Saxena wrote:
Hi all,
I have query regarding usage of host_byte DID_REQUEUE vs DID_RESET
returned by LLD to SCSI mid layer.
Let me give some background here.
I am using megaraid_sas controller. megaraid_sas driver returns all
outstanding SCSI commands back to SCSI layer with DID_RESET host_byte
before resetting controller.
The intent is- all these commands returned with DID_RESET before
controller reset should be retried by SCSI.
In few distros, I have observed that if SYNCHRONIZE_CACHE command(should
be applicable for all non Read write commands) is outstanding before
resetting controller and driver returns those commands back with
DID_RESET then SYNCHRONIZE_CACHE command not retried but failed to upper
layer
but other READ/WRITE commands were not failed but retried. I was running
filesystem IOs and SYNHRONIZE_CACHE returned with error cause filesystem
to go in READ only mode.
Later I checked and realized below commit was missing in that distro
kernel and seems to cause the problem-
a621bac scsi_lib: correctly retry failed zero length REQ_TYPE_FS commands
But distro kernel has below commit-
89fb4cd scsi: handle flush errors properly
Issue does not hit on the kernels which don't have both commits but hits
when commit- "89fb4cd scsi: handle flush errors properly " is there
but commit- "a621bac scsi_lib: correctly retry failed zero length
REQ_TYPE_FS commands" is missing.
This issue is observed with mpt3sas driver as well and should be
applicable to all LLDs returning non read write commands with DID_RESET.
Returning DID_REQUEUE instead of DID_RESET from driver solves the problem
irrespective of whether these above mentioned commits are there or not in
kernel.
So I am thinking to use DID_REQUEUE instead of DID_RESET in megaraid_sas
driver for all SCSI commands(not only limited to SYNCHRONIZE_CACHE or non
read write commands)
before resetting controller. As I mentioned intent is those outstanding
commands returned by driver before doing controller reset should be
retried and as soon as reset is complete,
driver will be processing those commands. There is no sense key associated
with SCSI commands returned.
I browsed SCSI code and get to know DID_REQUEUE causes command to be
retried by calling- scsi_queue_insert(cmd, SCSI_MLQUEUE_DEVICE_BUSY).
This seems to be good enough for our requirement of commands to be
re-tried by SCSI layer.
Please provide feedback if anyone forsee any issue with using DID_REQUEUE
for this use case.
I will be doing some testing with DID_REQUEUE in place of DID_RESET in
megaraid_sas driver.
I have attached here a proposed patch for megaraid_sas driver.
If there are no concerns, I will send this patch to SCSI mailing list.
Hmm.
DID_RESET is meant to be an indicator to the SCSI midlayer that the host
/ device was reset, and the command _should_ be retried.
DID_REQUEUE OTOH is an indicator to the SCSI midlayer to retry the command.
The problem with DID_RESET is that it's slightly underspecified; there
is no indication if the command has been processed (and if so, up to
which parts?)
DID_REQUEUE, OTOH, will always cause a retry.
So yeah, I guess DID_REQUEUE is a better choice here.
Cheers,
Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@xxxxxxx +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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