IBM DS3500 - Raid Level

7914Admin_Logical.fm Draft Document for Review March 28, 2011 12:24 pm
234 IBM System Storage DS3500: Introduction and Implementation Guide
Figure 9-11 Change Ownership confirmation
RAID level
Changing the RAID level of an Array is performed in a non disruptive manner. This feature is
called
Dynamic RAID Migration (DRM). The system remains fully operational while the
process takes place. A few possible reasons why customers may want to do this operation
are:
򐂰 The storage requirements changed over time and existing RAID levels are no longer
optimal for a particular environment.
򐂰 The performance tuning process indicates that a different RAID level is more appropriate
than the existing one.
It is possible to change any RAID level depending on some restrictions that apply to the new
arrays:
򐂰 RAID 1 or 10 requires an even number of disk drives.
򐂰 RAID 3 and 5 require at least three drives.
򐂰 RAID 6 requires at least five drives.
򐂰 There is a limit of 30 drives per array for RAID 3, 5, and 6 arrays.
There are also other limitations when there is not enough free space in the array. For
example, a RAID 5 array of four disk drives with no free space cannot be migrated directly to
RAID 1. If this migration is attempted, an error message will be displayed stating that there is
not enough free space. There must be enough free capacity to change the RAID level. Also, if
the array has an odd number of drives and a migration to RAID 1 is required, a disk must be
added to the array prior to performing the procedure.
When changing from RAID 1 to RAID 5, free space in the array can be gained, which can be
used to define new logical drives or expand existing ones.
When the procedure starts, it reorganizes the data segments in the array according to the
new RAID level, and a large amount of I/O happens, so there is an impact on performance
while the migration happens. The performance impact can be controlled to a certain extent by
changing the value of the modification priority. This parameter is set on a logical drive basis,
and it can be changed for one logical drive, for a set of them or for all logical drives in the
Array. See chapter 9.5.1, “Change Modification Priority” on page 245 for details.
Changing the modification priority to a low value during the migration process minimizes
performance degradation. When the migration finishes, the value can be increased to reduce
the time for a rebuild in case of a drive failure. This minimizes the critical time of
non-redundant operation caused by the disk drive fault.