Re: queue_depth tracking from LLD

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



The mid-layer queue depth handling is really designed/optimized around behavior for a JBOD. This, if it's a single-lun device, the LLDD could largely ignore doing anything
with adjusting the queue depth.

However, for arrays, with multiple luns, the queue depth is usually a target-level resource, so the midlayer/block-layer's implementation falls on its face fairly quickly. I brought this up 2 yrs ago at storage summit. What needs to happen is the creation of queue ramp-down and ramp-up policies that can be selected on a per-lun basis, and have these implemented in the midlayer (why should the LLDD ever look at scsi command results). What will make this difficult is the ramp-up policies, as it can be very target device-specific or configuration/load
centric.

In the meantime, if you look at any LLDD that is worth its salt, and it will be implementing it's own queue ramp-down and ramp-up algorithms internally. They will look for QUEUE_FULLs to ramp-down, and selecting a rate and methodology for the ramp-up. They will use this routine
to do the queue depth changing.

-- james s


Christof Schmitt wrote:
I just came across the SCSI midlayer function scsi_track_queue_full.

If a SCSI command is returned with a status of QUEUE_FULL, then this
is mapped to ADD_TO_MLQUEUE and "device blocked". So, there is already
a mechanism in place. Is a LLD driver expected to additionally call
something like this to decrease the queue depth?

if (status_byte(scmd->result) == QUEUE_FULL)
	scsi_track_queue_full(sdev, sdev->queue_depth - 1))

If a LLD does this, should it also increase the queue depth again when
no more QUEUE_FULL status are seen? To me this looks like a
duplication of the midlayer device blocking, but i assume there is a
reason in having both, scsi_track_queue_full and the device blocking.

--
Christof Schmitt
--
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

--
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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Index of Archives]     [SCSI Target Devel]     [Linux SCSI Target Infrastructure]     [Kernel Newbies]     [IDE]     [Security]     [Git]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Linux ATA RAID]     [Linux IIO]     [Samba]     [Device Mapper]
  Powered by Linux