Avago Technologies MegaRAID Fast Path Software - Manual (Page 29)

LSI Corporation Confidential | July 2011 Page 29
MegaRAID SAS Software User Guide Chapter 2: Introduction to RAID | Components and Features
The hot spare can be of two types:
 Global hot spare
 Dedicated hot spare
2.4.12.1 Global Hot Spare Use a global hot spare drive to replace any failed drive in a redundant drive group as
long as its capacity is equal to or larger than the coerced capacity of the failed drive. A
global hot spare defined on any channel should be available to replace a failed drive on
both channels.
2.4.12.2 Dedicated Hot Spare Use a dedicated hot spare to replace a failed drive only in a selected drive group. One or
more drives can be designated as a member of a spare drive pool. The most suitable
drive from the pool is selected for failover. A dedicated hot spare is used before one
from the global hot spare pool.
Hot spare drives can be located on any RAID channel. Standby hot spares (not being
used in RAID drive group) are polled every 60 seconds at a minimum, and their status
made available in the drive group management software. RAID controllers offer the
ability to rebuild with a disk that is in a system but not initially set to be a hot spare.
Observe the following parameters when using hot spares:
 Hot spares are used only in drive groups with redundancy: RAID levels 1, 5, 6, 10, 50,
and 60.
 A hot spare connected to a specific RAID controller can be used to rebuild a drive
that is connected only to the same controller.
 You must assign the hot spare to one or more drives through the controller BIOS or
use drive group management software to place it in the hot spare pool.
 A hot spare must have free space equal to or greater than the drive it replaces. For
example, to replace an 500-GB drive, the hot spare must be 500-GB or larger.
2.4.13 Disk Rebuilds When a drive in a RAID drive group fails, you can rebuild the drive by re-creating the
data that was stored on the drive before it failed. The RAID controller re-creates the
data using the data stored on the other drives in the drive group. Rebuilding can be
done only in drive groups with data redundancy, which includes RAID 1, 5, 6, 10, 50,
and 60 drive groups.
The RAID controller uses hot spares to rebuild failed drives automatically and
transparently, at user-defined rebuild rates. If a hot spare is available, the rebuild can
start automatically when a drive fails. If a hot spare is not available, the failed drive must
be replaced with a new drive so that the data on the failed drive can be rebuilt.
The failed drive is removed from the virtual drive and marked ready awaiting removal
when the rebuild to a hot spare begins. If the system goes down during a rebuild, the
RAID controller automatically resumes the rebuild after the system reboots.
NOTE: When the rebuild to a hot spare begins, the failed drive is often removed from
the virtual drive before management applications detect the failed drive. When this
occurs, the events logs show the drive rebuilding to the hot spare without showing the
failed drive. The formerly failed drive will be marked as “ready” after a rebuild begins to
a hot spare.