Enclosure Loss Protection Planning - IBM System Storage DS3500 Introduction And Implementation Manual

Draft Document for Review March 28, 2011 12:24 pm
Best practice: For best practice configuration it is recommended that you have one
hotspare for every 18 - 20 disk drives in the system.
When defining hot spare drives, you need to consider the following rules:
Hot spares must be of the same drive type (SAS or SAS nearline).
Hot spares must be of the same or larger size as the failed drive.
For example, if you use a mixture of different disk drive sizes in your arrays, the hot spare
drive size must be large enough so that it can effectively replace any of the drives of the same
type in the system that it is protecting. This also means that in a mixed drive type environment
you would need to have a minimum of two hot spares, one SAS and one SAS nearline.
The following methods are available to allocate hot spare drives in the storage subsystem:
Automatic assignment: The storage subsystem automatically calculates the number of hot
spare drives needed and allocates accordingly. This can be used on unconfigured storage
subsystems.
Explicit assignment: The hot spare drives are manually selected and assigned.
Use the IBM DS Storage Manager to configure the above options.

3.3.5 Enclosure loss protection planning

Enclosure loss protection will enable your system to be more resilient against hardware
failures. Enclosure loss protection means that you spread your protection arrays across
multiple enclosures rather than in one enclosure so that a failure of a single enclosure does
not take an array offline.
By default, the automatic configuration is enabled. However, this is not a best practice for the
method of creating arrays. Instead, we use the manual method, because this allows for more
configuration options to be available at creation time.
Best practice: Manual array configuration allows for greater control over the creation of
arrays.
Figure 3-20 on page 62 shows an example of the enclosure loss protection. If enclosure
number 2 fails, the array with the enclosure loss protection can still function (in a degraded
state), because the other drives are not affected by the failure.
Chapter 3. IBM System Storage DS3500 Storage System planning tasks
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