Apple Xsan 1.0 - Manual (Page 23)

Chapter 2 Before You Begin 23
If high availability is important, you should use at least two controllers, one as the
primary controller and one as a standby. You can specify additional controllers as
needed, and set their failover priorities to determine the order in which they are tried if
the primary controller stops responding.
Choosing Standby Controllers
To be sure that SAN volumes are always available, set up at least one standby controller
that can take over if your primary metadata controller fails. A standby controller also
makes it possible for you to upgrade software on the controllers without interrupting
user access to SAN volumes.
Combining Clients and Controllers
The same computer can function as both a metadata controller and a client. It’s
possible, for example, to set up a SAN consisting of a single Xserve RAID and one
computer that acts as both controller and client. Any computer you specify as a
controller can also act as a client.
If, for example, you don’t have a computer to dedicate as a standby controller, you can
assign a computer that is normally used as a client to take over controller duties if the
primary controller fails.
To keep clients and controllers separate, you can set up client-only computers for your
users.
Choosing Where to Store Metadata and Journal Data
The metadata and journal data that describe a volume are not stored on the volume’s
metadata controller but on the volume itself. By default, they are stored on the first
storage pool in the volume. If the volume consists of more than one storage pool, you
can choose which storage pool is used to store metadata and journal data.
In most cases, storing metadata and journal data on the same storage pool as user data
results in good performance. However, for the best possible performance, consider
storing metadata and journal data on separate storage pools within the volume.
Choosing an Allocation Strategy
The allocation strategy you choose for a volume determines the order in which its
storage pools are filled with data. You can choose round robin, fill, or balance.
If you choose round robin, Xsan writes new data in turn to each storage pool in the
volume.
If you choose fill, Xsan writes all new data to the first storage pool in the volume until
that storage pool is full, then moves to the next storage pool. This is a good choice if
you want to keep a particular storage pool unused as long as possible.
If you choose balance, Xsan writes new data to the storage pool with the most free
space.
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