Dell POWERVAULT MD3600I - Manual (Page 83)

Enclosure Loss Protection
Enclosure loss protection is an attribute of a disk group. Enclosure loss protection guarantees accessibility to the data
on the virtual disks in a disk group if a total loss of communication occurs with a single expansion enclosure. An
example of total loss of communication may be loss of power to the expansion enclosure or failure of both RAID
controller modules.
CAUTION: Enclosure loss protection is not guaranteed if a physical disk has already failed in the disk group. In this
situation, losing access to an expansion enclosure and consequently another physical disk in the disk group
causes a double physical disk failure and loss of data.
Enclosure loss protection is achieved when you create a disk group where all of the physical disks that comprise the
disk group are located in different expansion enclosures. This distinction depends on the RAID level. If you choose to
create a disk group by using the Automatic method, the software attempts to choose physical disks that provide
enclosure loss protection. If you choose to create a disk group by using the Manual method, you must use the criteria
specified below.
RAID Level Criteria for Enclosure Loss Protection
RAID level 5 or RAID
level 6 Ensure that all the physical disks in the disk group are located in different expansion
enclosures.
Because a RAID level 5 requires a minimum of three physical disks, enclosure loss protections
cannot be achieved if your storage array has less than three expansion enclosures. Because a
RAID level 6 requires a minimum of five physical disks, enclosure loss protections cannot be
achieved if your storage array has less than five expansion enclosures.
RAID level 1 Ensure that each physical disk in a mirrored pair is located in a different expansion enclosure.
This enables you to have more than two physical disks in the disk group within the same
expansion enclosure.
For example, if you are creating a six physical disk, disk group (three-mirrored pairs), you could
achieve enclosure loss protection with only two expansion enclosures by specifying that the
physical disk in each mirrored pair are located in separate expansion enclosures. This example
shows this concept:
• Mirror pair 1 — Physical disk in enclosure 1 slot 1 and physical disk in enclosure 2 slot
1.
• Mirror pair 2 — Physical disk in enclosure 1 slot 2 and physical disk in enclosure 2 slot
2.
• Mirror pair 3 — Physical disk in enclosure 1 slot 3 and physical disk in enclosure 2 slot
3.
Because a RAID level 1 disk group requires a minimum of two physical disks, enclosure loss
protections cannot be achieved if your storage array has less than two expansion enclosures.
RAID level 0 Because RAID level 0 does not have consistency, you cannot achieve enclosure loss
protection.
Drawer Loss Protection
In expansion enclosures that contain drawer-based physical disks, a drawer failure can prevent access to data on the
virtual disks of a disk group.
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