Mylex Disk Array controller Software Kit 2.01, Installation Manual And User'S Manual

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Appendix E
The MYLEXDRV.SIZ File
Drive Size Limits Overview
One of the common characteristics of disk drives grouped into disk arrays or
RAID groups is that each disk drive in the logical group is generally the
same size (capacity). While it is not strictly required that this be so, it is most
beneficial for the following reasons:
• If physical disk drives defined in a logical group are of different sizes,
the drive initialization process sees to it that the usable capacity of each
physical disk drive will be the same. This can lead to capacity being
unused or unavailable for use, and wasted capacity usually means
wasted cost.
EXAMPLE: If three physical drives are of capacity 3 GB, 3 GB, and 4
GB respectively, and they are grouped together into one RAID 5 logical
drive, initialization will result in three drives using 3 GB of capacity
each. One GB on the 4 GB drive is unused.
• If hot spare (standby drives) are to be used in a configuration to take
over for a failed drive, the hot spare drive’s capacity must be equal to or
greater than the capacity of the smallest drive in the configuration.
Therefore, using physical disk drives of different capacities can
actually interfere with proper drive rebuild operation.
When using multiple physical disk drives from the same drive vendor, it’s
usually not difficult to ensure that the drives to be used together in a logical
group all have the same capacity. The challenge arises when attempting to
use disk drives of similar capacity from multiple disk drive vendors.
For example, a 4 GB disk drive from a particular vendor may actually be
4.01 GB. A 4 GB drive from another vendor may actually be 4.12 GB. Even
though the drives capacities are similar, they are not exact, and the problems
described above can occur. If the 4.01 GB drive from Vendor A is used as a
hot spare in a RAID 5 array consisting of 4.12 GB drives from Vendor B, the
rebuild process will fail because the hot spare drive is smaller than the size of
the drive it’s trying to replace.