VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

How Hot-Relocation works
228 VERITAS Volume Manager Administrator’s Guide
How Hot-Relocation works
Hot-relocation allows a system to react automatically to I/O failures on redundant
(mirrored or RAID-5) VxVM objects, and to restore redundancy and access to those
objects. VxVM detects I/O failures on objects and relocates the affected subdisks to disks
designated as spare disks or to free space within the disk group. VxVM then reconstructs
the objects thatexisted before the failureand makesthem redundant and accessibleagain.
When a partial disk failure occurs (that is, a failure affecting only some subdisks on a
disk), redundant data on the failed portion of the disk is relocated. Existing volumes on
the unaffected portions of the disk remain accessible.
Note Hot-relocation is only performed for redundant(mirrored or RAID-5) subdisks on a
failed disk. Non-redundant subdisks on a failed disk are not relocated, but the
system administrator is notified of their failure.
Hot-relocation is enabled by default and takes effect without the intervention of the
system administrator when a failure occurs.
Thehot-relocationdaemon, vxrelocd,detects andreacts to VxVMeventsthat signify the
following types of failures:
◆ disk failure—this is normally detected as a result of an I/O failure from a VxVM
object. VxVM attempts to correct the error. If the error cannot be corrected, VxVM
tries to access configuration information in the private region of the disk. If it cannot
access the private region, it considers the disk failed.
◆ plex failure—this is normally detected as a result of an uncorrectable I/O error in the
plex (which affects subdisks within the plex). For mirrored volumes, the plex is
detached.
◆ RAID-5 subdisk failure—this is normally detected as a result of an uncorrectable I/O
error. The subdisk is detached.
When vxrelocd detects such a failure, it performs the following steps:
1. vxrelocd informs the system administrator (and other nominated users, see
“Modifying the Behavior of Hot-Relocation” on page 243) by electronic mail of the
failureandwhich VxVM objectsareaffected.See “Partial DiskFailureMail Messages”
on page 231 and “Complete Disk Failure Mail Messages” on page 232 for more
information.
2. vxrelocd next determines if any subdisks can be relocated. vxrelocd looks for
suitable space on disks that have been reserved as hot-relocation spares (marked
spare) in the disk group where the failure occurred. It then relocates the subdisks to
use this space.