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In this Document

Symptoms

Cause

Solution

Pool layout

ZIL (ZFS Intent Log)

Volume block size

Partition alignment

TCP connections

Number of iSCSI sessions per iqn target

References

Applies to:

Symptoms

In some circumstances, some iSCSI latencies might be discernible on iSCSI client side. This document provides some typical checks for iSCSI performance issues.

Cause

Among many possible causes, the following are of interest :

• pool layout of datapool selected(raidz2, mirror, stripe)
• availability of intent log devices (ZIL)
• volblock size of the volume
• partition alignment from client side
• TCP connections
• Number of iSCSI sessions per iqn target

Solution

Pool layout

To be able to provide a good level of IOPs, the pool layout to use is mirror. Use raidz2 only if the IO workload is similar to video stream or related to big file access.

ZIL (ZFS Intent Log)

• All zpools have a ZIL even if a specific log device is not defined. If there is no dedicated log device, the ZIL is made from the data area of a pool. This can double the number of actual writes. A sync write would first be written to the ZIL on the drives and then commited to the normal data area. This "two for one" writes can have a extremely large impact on performance.To avoid this "two for one" penalty it is a good idea to separate the ZIL from the data drives and put it on a device with higher performance capabilities. A fast SSD is used for this purpose on the 7000 arrays : the logzilla.
• If the host uses the iSCSI LUNs in ZFS volumes, the write cache can be enabled on the NAS head side. Thanks to this, the SCSI ACK will be sent back immediately to the host at high response time. If the host uses write cache but not through ZFS volumes, there is a risk of data loss in case of power loss on the NAS head. If integrity is a priority, always use logzilla. For better results, use 2 logzilla per pool (at least) in a stripe layout.

Volume block size

The volblocksize specifies the block size of a volume (iSCSI). The block size cannot be changed once a volume has been written, so set the block size at volume creation time. The default block size for volumes is 8 Kbytes. It can be set up to 128Kbytes.
Check what is the IO block size on host side before creating any iSCSI LUN.
Even if the VmWare block size is 1MB minimum, the genuine IO blocksize from the embedded application may be smaller.

Partition alignment

This topic is detailed on a blog from David Lutz.
With proper alignment, a single client block that is the same size or less as the volume block size of a LUN will be contained entirely within a single volume block in the LUN. Without proper alignment, that same client block may span multiple volume blocks in the LUN. That could result in 2 appliance reads for a single client read, and 2 appliance reads plus 2 appliance writes for a single client write. This will obviously have a big impact on performance if ignored.
For details, see Partition Alignment Guidelines for Unified Storage.

TCP connections

There is a possible cause for iSCSI timeout error messages occurring on client side. On the NAS boxes, it is considered that a max of 300 iSCSI sessions can be handled correctly. In iSCSI terminology, a 'session' is a single logical connection between an iSCSI initiator and an iSCSI target, and a 'connection' is a TCP connection. Each session can have multiple TCP connections for multipathing purposes (see http://utcc.utoronto.ca/~cks/space/blog/tech/ISCSIMCSAdvantages).
A common way to fall in a big amount of iSCSI sessions is to provide only 1 LUN per iqn target. If many LUNs are configured, this will increase the number of iSCSI sessions. To reduce this, just assign more LUNs to less iqn targets.

Number of iSCSI sessions per iqn target

Under some conditions it is possible to increase the number of iSCSI sessions per iSCSI target. On Solaris host, do as follows :

Back to <Document 1331769.1> Sun Storage 7000 Unified Storage System: How to Troubleshoot Performance Issues

See NAS wiki page for more info: https://stbeehive.oracle.com/teamcollab/wiki/AmberRoadSupport:iSCSI+is+hung+or+very+slow

References

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