On 06/09/2010 09:19 AM, Patrick J. LoPresti wrote:
Hi, Mike, and thank you for your reply.
On Tue, Jun 8, 2010 at 10:00 PM, Mike Christie<[email protected]> wrote:
# To control how many commands the session will queue set
# node.session.cmds_max to an integer between 2 and 2048 that is also
# a power of 2. The default is 128.
node.session.cmds_max = 128
# To control the device's queue depth set node.session.queue_depth
# to a value between 1 and 1024. The default is 32.
node.session.queue_depth = 32
Let me see if I understand...
In this context, "device" means "Linux block device". The kernel's
filesystem layer sends SCSI commands to the block device, and
"queue_depth" is the maximum number of outstanding (i.e.
not-yet-completed) commands the kernel will allow.
In this case, the block device is implemented by the iSCSI driver,
which forwards the SCSI commands over the session, which is basically
the client-to-server iSCSI socket. "cmds_max" is the maximum number
of outstanding (i.e., not-yet-completed) commands that the iSCSI
driver will permit for the session.
Yeah, the session is the connection from the initiator/iscsi-driver to
the target portal. For this we have a socket. The session can access
multiple devices/luns through this session. When you run iscsiadm -m
sesssion -P 3 you will see the session and then underneath it the
devices/luns accessed through it.
Is this correct so far?
If so, I have a couple more questions.
First, why would I want cmds_max larger than queue_depth? Is it
something to do with the protocol (e.g., a single command to the
device can become multiple commands in the session)? Or is it just
because one session can have multiple targets, which means multiple
block devices?
Yeah on the updated second one. A session can access multiple logical
units. It depends on the target. Some targets will match a target to a
lun, others will present X targets with Y LUs accessed through each target.
Second, how does the number of commands relate to the amount of data
being transferred? Put another way, how much data can a single
command request from the disk?
It depends. At the block layer there is a
/sys/block/sdX/queue/max_sectors_kb which limits the size of the command
that can be made. However, there a lot of other things that can prevent
us from making IOs that large.
At the iscsi level then we break up the scsi command into multiple iscsi
packets. These are 48 bytes when not using digests and then the data
part of the packet is limited by the MaxRecvDataSegmentLength the target
and initiator have negotiated for on their sides. And actually for the
initial packet of a write command the FirstBurstLength and ImmediateData
come into play.
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