Intel 170 Servers, AS/400 RISC Server manual Extended Write Operations, ISCSI Disk I/O Operations

yWrite Cache Property

When the disk device write cache property is disabled, disk operations have similar performance characteristics to shared disks. You may examine or change the “Write Cache” property on Windows by selecting disk “properties” and then the “Hardware tab”. Then view “Properties” for a selected disk and view the “Disk Properties” or “Device Options” tab.

All dynamically and statically linked storage spaces have “Write Cache” enabled by default. Shared links have “Write Cache” disabled by default. While it is possible to enable “Write Cache” on shared disks, we recommend to keep it disabled to insure integrity during clustering fail-over operations.

There is also negligible performance benefit to enabling the write cache on shared disks.

yExtended Write Operations

Even though a Windows disk driver may have write cache enabled, the system or applications consider some write operations sensitive enough to request extended writes or flush operations “write through” to the disk. These operations incur the higher CPW cost regardless of the write caching property.

yFor the IXS and IXA solutions - do not enable the disk driver “Enable advanced performance” property provided in Windows 2003. When enabled, all extended writes are turned into normal cached operations and flush operations are masked. This option is only intended to be used when the integrity of the write operations can be guaranteed via write through or battery backed memory. The IXS/IXA with write caching enabled cannot make this guarantee.

yIXS/IXA Disk Capacity Considerations

The level of disk I/O achieved on the IXS or IXA varies depending on many variables, but given an adequate storage subsystem, the upper cap on I/O for a single server is limited by the IXS/IXA IOP component. Except in extreme test loads, it’s unlikely the IOP will saturate due to disk activity.

When multiple IXS/IXA servers are attached under the same System i partition, the partition software imposes a cap on the aggregate total I/O from all the servers. It is not a strict limitation, but a typical capacity level is approximately 6000 to 10000 disk operations/sec.

17.2.2 iSCSI Disk I/O Operations:

yThe iSCSI disk operations use a more scalable storage I/O access architecture than the IXS and IXA solutions. As a result, a single integrated server can scale to greater capacity by using multiple target and initiator iSCSI HBAs to allow multiple data paths.

yIn addition, there is no inherent partition cap to the iSCSI disk I/O. The entire performance capacity of installed disks and disk IOAs is available to iSCSI attached servers.

•The Windows disk drive “write cache” policy does not directly affect iSCSI operations. Write operations always “write through” to the host disk IOAs, which may or may not cache in battery backed memory (depending on the capabilities and configuration of the disk IOA).

yiSCSI attached servers use non-reserved System i virtual storage in order to perform disk input or output. Thus, disk operations use host memory as an intermediate read cache. Write operations are flushed to disk immediately, but the disk data remains in memory and can be read on subsequent operations to the same sectors.

While the disk operations page through a memory pool, the paging activity is not visible in the “Non-DB” pages counters displayable via the WRKSYSSTS command. This doesn’t mean the memory is not actively used, it’s just difficult to visualize how much memory is active. WRKSYSSTS will show faults and paging activity if the memory pool becomes constrained, but some write operations also result in faulting activity.

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
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