IBM System Storage DS4000, Руководство по оборудованию

Страница: 134 / 566

116
IBM Midrange System Storage Hardware Guide
Quantity and type of hot-spares drives
The is no fixed rule about the quantity of disk drives to assign as hot spares, but as a general
rule about disk usage and availability, we recommend defining one of every 30 drives of a
particular media type and interface type, or one for every two fully populated enclosures.
Because of disk sizes, and especially in large configurations with arrays containing numerous
drives, the reconstruction of a failed drive to a hot spare drive can take a long time,
proportional to the quantity of drives in the array and the size of the disks. If in addition to that
time, you have to wait to have a new disk available onsite to replace the failed drive, then the
probability of having another disk failure increases. Having multiple spare drives will not
mitigate the reconstruction time, but at least an array will be prepared sooner for a second
disk failure.
4.1.4 Storage partitioning
Storage partitioning adds a high level of flexibility to the DS5000 storage subsystem. It
enables you to connect multiple and heterogeneous host systems to the same storage
subsystem, either in stand-alone or clustered mode. The term storage partitioning is
somewhat misleading, as it actually represents a host or a group of hosts and the logical
disks they access.
Without storage partitioning, the logical drives configured on a DS5000 storage subsystem
can only be accessed by a single host system or by a single cluster. This can lead to
inefficient use of storage subsystem hardware unless the use of the DS5000 storage
subsystem is dedicated to a single host (for example, SVC attachment, where it is seen as a
single host).
Storage partitioning, on the other hand, allows the creation of “sets”, containing the hosts with
their host bus adapters and the logical drives. We call these sets
storage partitions
. The host
systems can only access their assigned logical drives, just as though these logical drives
were locally attached to them. Storage partitioning adapts the SAN idea of globally accessible
storage to the local-storage-minded operating systems.
Storage partitioning allows mapping and masks the logical device or LUN (that is why it is
also referred to as LUN masking). This means that after the LUN is assigned to a host, it is
hidden from all other hosts connected to the same storage subsystem. Therefore, access to
that LUN is exclusively reserved for that host.
It is a good practice to configure storage partitioning prior to connecting multiple hosts.
Operating systems such as AIX or Windows will write their signatures to any device it can
access.
Heterogeneous host support means that the host systems can run different operating
systems. But be aware that all host systems within a particular storage partition have
unlimited access to all logical drives in this partition. Therefore, file systems on these logical
drives must be compatible with host systems. To ensure that this is true, it is best to run the
same operating system on all hosts within the same partition. Some operating systems might
be able to mount foreign file systems.
Note: There is no definitive recommendation about how many hot spares you should
install, but it is common practice to use a ratio of one hot spare for about 30 drives.