SCST Performance
================

A question that is asked often is how to tune performance. This means how to
improve the IOPS and/or bandwidth measured at the initiator side. In this
README it is explained how to optimize storage performance.

Local storage
-------------
Start with measuring the performance of the local block device(s). IOPS can be
measured e.g. as follows:

    fio --ioengine=libaio --rw=randread --ioscheduler=none --numjobs=$(nproc) \
    --runtime=60 --group_reporting=1 --gtod_reduce=1 --norandommap \
    --thread --buffered=0 --iodepth=256 --iodepth_batch=128 --bs=4k \
    --name=bdev --filename=/dev/...

The bandwidth supported by a block device can be measured by increasing the
block size to a larger value in the above command, e.g. --bs=1M.

Storage network
---------------
Start with measuring the network bandwidth using your favorite tool, e.g.
netperf for non-RDMA networks or ib_write_bw for RDMA networks.

Next, add a nullio LUN to SCST, e.g. by adding the following in /etc/scst.conf:

    HANDLER vdisk_nullio {
        DEVICE disk09 {
            blocksize 4096
            size_mb   256
        }
    }

    TARGET_DRIVER ... {
        TARGET ... {
	    LUN ... disk09
	}
    }

After a nullio LUN has been added, verify that this LUN is visible at the
initiator side. If it is not visible at the initiator side, consider
rescanning LUNs or disconnecting and reconnecting the initiator system to the
SCST server.

Once the nullio LUN is visible at the initiator side, measure IOPS and
bandwidth. The bandwidth should be close to the network bandwidth. When using
the Linux iSCSI initiator this may require configuring multiple iSCSI sessions
and activating multipathd on top of the multiple iSCSI sessions. The sequence
for logging in with iSCSI and activating multiple iSCSI sessions is as follows:

    iscsiadm -m iface -I iface2 -o new
    iscsiadm -m iface -I iface2 -o update -n iface.initiatorname -v ${iqn2}
    iscsiadm -m discovery -t st -p ${scst_ip_address}
    iscsiadm -m discovery -t st -p ${scst_ip_address} -I iface2
    iscsiadm -m node -p ${scst_ip_address} -l
    iscsiadm -m node -p ${scst_ip_address} -I iface2 -l

SCST Configuration
------------------
If the number of IOPS measured at the initiator side is significantly lower
than the minimum of the IOPS supported by the local storage and the storage
network, further tuning is required. Look up in /proc/interrupts which CPU
cores process the most network and storage interrupts and configure the SCST
kernel threads such that these run on other CPU cores than those that process
the most interrupts by configuring the cpu_mask attribute. More information
about the SCST cpu_mask sysfs attribute is available in the SCST README.