Xsan 2 Administrator Guide - Adjusting RAID System Performance Settings

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Adjusting RAID System Performance Settings

RAID system performance settings, which affect parameters such as drive caching,
RAID controller caching, and read prefetching, can have a significant effect on Xsan
volume performance. Follow these guidelines.

Enable Drive Caching

In addition to the caching performed by the RAID controller, each drive in an array can
perform caching at the drive level to improve performance.


If you enable drive caching for a RAID set, make sure the system is

connected to an uninterruptible power supply (UPS). Otherwise, you could lose
cached data if the power fails.

Enable RAID Controller Write Caching

Without RAID controller write caching, a request to write data to the associated LUN
isn’t considered finished until the data is written to the physical disks that make up
the array. Only then can the next write request be processed. (This is sometimes called
“write-through caching.”)

When RAID controller write caching is enabled, a request to write data is considered
finished when the data is in the cache. This is sometimes called “write-back caching.”
Write requests are processed more quickly because the file system only needs to write
to the fast cache memory and doesn’t need to wait for the slower disk drives.

Be sure to enable write caching on RAID controllers that support metadata storage

Although some large write requests might benefit from caching, often they don’t.
By placing a volume’s metadata storage pool on a RAID controller separate from the
data storage pools, you can enable caching on the RAID controller used for metadata
and disable caching on the RAID controller used for data.

When the file system is relying on caching in this way, you must guarantee that
data in the cache isn’t lost before it’s written to disk. Data written to disk is safe if the
power fails, but data in a cache is not. To be sure that a power failure can’t cause the
loss of cached data, protect your RAID systems with RAID controller backup batteries
or a UPS.


If you enable controller write caching on a RAID system, make sure the

system includes controller backup batteries and, preferably, is connected to a UPS.

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Enable Read Prefetching

Read prefetching is a technique that improves file system read performance when
data is being read sequentially, as in the case of audio or video streaming, for example.

When read prefetching is enabled, the RAID controller assumes that a read request for
a block of data will be followed by requests for adjacent data blocks. To prepare for
these requests, the RAID controller reads the requested data and the following data,
and stores it in cache memory. Then, if the data is requested, it’s retrieved from the fast
cache instead of from the slower disk drives.


Chapter 2

Planning a Storage Area Network

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Follow step by step instructions for setting up a shared

volume on an Xsan SAN.

This chapter explains how to connect the SAN networks, prepare RAID arrays (LUNs),
use the Xsan Admin application, set up a SAN, and create a shared volume. This
chapter also tells you how to administer Xsan remotely, rename a SAN, remove a SAN,
set up additional SANs, and manage multiple SANs.

Connecting Computers and Storage Devices

Before you open Xsan Admin to configure your SAN, you must connect client
computers, controller computers, and storage devices to the SAN’s Fibre Channel
and Ethernet networks. In addition, make sure your networks meet the requirements
summarized in “Fibre Channel Fabric” on page 22 and “Ethernet TCP/IP Network” on
page 23.

Preparing LUNs

New RAID systems often come configured as one or more RAID arrays. So, out of the
box, your RAID system might provide LUNs that you can use for most SAN applications.
For details, see the documentation for your RAID system. Unless you have well-defined,
special needs, no other LUN preparation is needed.

To set up other combinations of RAID arrays or slices, use the management application
that comes with your RAID systems to create the arrays before you add the resulting
LUNs to your SAN’s storage pools. For information about choosing a RAID scheme,
see “Choosing RAID Schemes for LUNs” on page 28.

Note: Don’t use Disk Utility to format arrays or slices for use with Xsan. LUNs are
labeled and initialized when you add them to a storage pool using Xsan Admin.
After they are labeled, the LUNs can’t be modified using Disk Utility.

Be sure to create arrays of the same size if you plan to add them to the same storage
pool. For more information, see “Assigning LUNs to Affinity Tags” on page 29.