Managing initiator requests

Why you need to manage initiator requests

Each physical port on the target HBA in the filer has a fixed number of command blocks for incoming initiator requests. When initiators send large numbers of requests, they can monopolize the command blocks and prevent other initiators from accessing the command blocks at that port.

With an igroup throttle, you can perform the following tasks:

Limit the number of concurrent I/O requests an initiator can send to the filer
Prevent initiators from flooding a port and preventing other initiators from accessing a LUN
Ensure that specific initiators have guaranteed access to the queue resources.

How Data ONTAP implements initiator request management

When you use igroup throttles, Data ONTAP calculates the total amount of command blocks available and allocates the appropriate number to reserve for an igroup based on the percentage you specify when you create a throttle for that igroup. Data ONTAP does not allow you to reserve more than 99% of all the resources. The remaining command blocks are unreserved and are available for use by igroups without throttles.

How to manage initiator requests

You use igroup throttles to specify what percentage of the queue resources they can reserve for their use. For example, if you set an igroup's throttle to be 20%, 20% of the queue resources available at the filer's ports are reserved for the initiators in that igroup. The remaining 79% of the queue resources is unreserved. In another example, if you have four hosts and they are in separate igroups, you might set the igroup throttle of the most critical host at 30%, the least critical at 10%, and the remaining two at 20%, leaving 19% of the resources unreserved.

How to use igroup throttles

When you create igroup throttles, you can use them to ensure that critical initiators are guaranteed access to the queue resources and that less critical initiators are not flooding the queue resources. You can perform the following tasks.

Create one igroup throttle per igroup (if desired; it is not required)
Note
Igroups without a throttle share all the unreserved queue resources.
Assign a specific percentage of the queue resources on each physical port to the igroup
Reserve a minimum percentage of queue resources for a specific igroup
Restrict an igroup to a maximum percentage of use
Allow an igroup throttle to exceed its limit by borrowing resources from
- The pool of unreserved resources to handle unexpected I/O requests
- The pool of unused reserved resources, if those resources belong to an igroup throttle whose igroup throttle throttle_borrow setting is set to y

Creating up an igroup throttle

To create an igroup throttle, complete the following step.

Step	Action
1	Enter the following command: *igroup set igroup_name* throttle_reserve percentage** Example `igroup set solaris-igroup1 throttle_reserve 20` Result The igroup throttle is created for solaris-igroup1, and it will persist through reboots.

Destroying an igroup throttle

To destroy an igroup throttle, complete the following step.

Step	Action
1	Enter the following command: *igroup set igroup_name* throttle_reserve 0**

Allowing or disallowing an igroup to borrow resources

To allow or disallow an igroup to borrow queue resources from the unreserved pool, complete the following step with the appropriate option (yes or no). The default when you create an igroup throttle is no.

Step	Action
1	Enter the following command: *igroup set igroup_name* throttle_borrow {yes \| no }** Example `igroup set solaris-igroup1 throttle_borrow y` Result When you set the `throttle_borrow` setting to `y`, the percentage of queue resources used by the initiators in the igroup might be exceeded if resources are available.

Step

Action

Enter the following command:

igroup set igroup_name throttle_borrow {yes | no }

Example

igroup set solaris-igroup1 throttle_borrow y

Result

When you set the throttle_borrow setting to y, the percentage of queue resources used by the initiators in the igroup might be exceeded if resources are available.

Displaying throttle information

To display information about the throttles assigned to igroups, complete the following step. The exceeds column displays the number of times the initiator sends more requests than the throttle allows. The borrows column displays the number of times the throttle is exceeded and the filer uses queue resources from the unreserved pool. In the borrows column, "N/A" indicates that the igroup throttle_borrow option is set to no.

Step	Action
1	Enter the following command: igroup show -t Result name reserved exceeds borrows solaris-igroup1 20% 0 N/A solaris-igroup2 10% 0 0

Displaying igroup throttle usage

To display real-time information about how many command blocks the initiator in the igroup is using (the first number under the port name) and the number of command blocks reserved for the igroup on the specified port (the second number under the port name), complete the following step.

Step	Action
1	Enter the following command: *igroup show -t -i number_of_seconds* [-a]** -`a` displays all throttles, including ones that are not using any command blocks. Example `igroup show -t -i 1` Result The following is a sample display: name reserved 4a 4b 5a 5b igroup1 20% 45/98 0/98 0/98 0/98 iqroup2 10% 0/49 0/49 17/49 0/49 unreserved 87/344 0/344 112/344 0/344 In this example, the display indicates that igroup1 is using 45 of the 98 reserved command blocks on adapter 4a, and igroup2 is using 17 of the 49 reserved command blocks on adapter 5a. Igroups without throttles are counted as unreserved.

Step

Action

Enter the following command:

igroup show -t -i number_of_seconds [-a]

-a displays all throttles, including ones that are not using any command blocks.

Example

igroup show -t -i 1

Result

The following is a sample display:

     name   reserved      4a      4b       5a       5b

igroup1           20%    45/98   0/98      0/98    0/98

iqroup2           10%     0/49   0/49     17/49    0/49

unreserved              87/344   0/344   112/344   0/344

In this example, the display indicates that igroup1 is using 45 of the 98 reserved command blocks on adapter 4a, and igroup2 is using 17 of the 49 reserved command blocks on adapter 5a.

Igroups without throttles are counted as unreserved.

Displaying LUN statistics on exceeding throttles

To display statistics about I/O requests for LUNs that exceed the igroup throttle, complete the following steps.

Step	Action
1	Display the QFULL column with LUN statistics, which indicates the number of initiator requests that exceeded the number allowed by the igroup throttle, and, as a result, received the QFULL message, by entering the following command: lun stats -o -i time_in_seconds Example `lun stats -o -i 1`
2	Display the total count of QFULLS sent for each LUN by entering the following command: lun stats -o

Step

Action

Display the QFULL column with LUN statistics, which indicates the number of initiator requests that exceeded the number allowed by the igroup throttle, and, as a result, received the QFULL message, by entering the following command:

lun stats -o -i time_in_seconds

Example

lun stats -o -i 1

Display the total count of QFULLS sent for each LUN by entering the following command:

lun stats -o

How a cluster failover affects igroup throttles

Igroup throttles manage physical ports, so during a cluster failover, their behavior varies according to the FCP cfmode that is in effect, as shown in the following table.

FCP cfmode	How igroup throttles behave when failover occurs
Standby	Throttles apply to the A ports: A ports have local throttles B ports have partner throttles
Partner	Throttles apply to the appropriate ports: A ports have local throttles B ports have partner throttles
Mixed	Throttles apply to all ports, and are divided by two

FCP cfmode

How igroup throttles behave when failover occurs

Standby

Throttles apply to the A ports:

A ports have local throttles
B ports have partner throttles

Partner

Throttles apply to the appropriate ports:

A ports have local throttles
B ports have partner throttles

Mixed

Throttles apply to all ports, and are divided by two

Displaying igroup throttle usage after takeover

To display information about how many command blocks the initiator in the igroup is using and the number of command blocks reserved for the igroup on the specified port after a takeover occurs, complete the following step.

Step	Action
1	Enter the following command: igroup show -t Example `igroup show -t` Result The following is a sample display: name reserved exceeds borrows solaris-igroup1 20% 0 N/A (Reduced by takeover to 10%) solaris-igroup2 10% 0 0 (Reduced by takeover to 5%)

Step

Action

Enter the following command:

igroup show -t

Example

igroup show -t

Result

The following is a sample display:

           name    reserved    exceeds    borrows

solaris-igroup1         20%         0         N/A  (Reduced by takeover to 10%)

solaris-igroup2         10%         0           0  (Reduced by takeover to  5%)