IBM N series Hardware Manual page 261

– Depending on the device configuration, paging to a SAN device might be slower than
paging to local storage. This issue is unlikely because paging operations benefit from
the write cache and multiple disk spindles that are available from enterprise-class SAN
storage systems. These benefits far outweigh the latency that is induced by a storage
networking transport unless the storage is oversubscribed.
– Sensitivity to bus resets can cause systems to become unstable. However, bus resets
do not generally affect all systems that are connected to the SAN. Microsoft
implemented a hierarchical reset handling mechanism within its STORport drivers for
Windows Server 2003 to address this behavior.
– High latency during pagefile access can cause systems to fail with a STOP message
(blue screen) or perform poorly. Carefully monitor the disk array to prevent
oversubscription of the storage, which can result in high latency.
– Some administrators that are concerned about paging performance might opt to keep
the pagefile on a local disk while storing the operating system on an N series SAN.
There are issues with this configuration as well.
– If the pagefile is moved to a drive other than the boot drive, system and crash memory
dumps cannot be written. This can be an issue when you are trying to debug operating
system instability in the environment.
– If the local disk fails and is not mirrored, the system fails and cannot boot until the
problem is corrected.
In addition, do not create two pagefiles on devices with different performance profiles,
such as a local disk and a SAN device. Attempting to distribute the pagefile in this manner
might result in kernel inpage STOP errors.
In general, if the system is paging heavily, performance suffers regardless of whether the
pagefile is on a SAN device or local disk. The best way to address this problem is to add
more physical memory to the system or correct the condition that is causing severe
paging. At the time of this writing, the costs of physical memory are such that a small
investment can prevent paging and preserve the performance of the environment.
It is also possible to limit the pagefile size or disable it completely to prevent SAN resource
contention. If the pagefile is severely restricted or disabled to preserve performance,
application instability is likely to result in cases where memory is fully used. Use this option
only for servers that have enough physical memory to cover the anticipated maximum
requirements of the application.
Microsoft Cluster Services and SCSI port drivers: the Microsoft Cluster Service uses
bus-level resets in its operation. It cannot isolate these resets from the boot device.
Therefore, installations that use the SCSIport driver with Microsoft Windows 2000 or 2003
must use separate HBAs for the boot device and the shared cluster disks. In deployments
where full redundancy is wanted, a minimum of four HBAs are required for MPIO. In Fibre
Channel implementations, employ zoning to separate the boot and shared cluster HBAs.
Chapter 18. Boot from SAN
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