VERITAS Volume Manager 3.5 Troubleshooting Guide (August 2002)

Failures on RAID-5 Volumes
6 VERITAS Volume Manager Troubleshooting Guide
Failures on RAID-5 Volumes
Failures are seen in two varieties: system failures and disk failures. A system failure means
that the system has abruptly ceased to operate due to an operatingsystem panic orpower
failure.Disk failuresimply that the dataon somenumber of disks hasbecome unavailable
due to a system failure (such as a head crash, electronics failure on disk, or disk controller
failure).
System Failures
RAID-5 volumes are designed to remain available with a minimum of disk space
overhead, if there are disk failures. However, many forms of RAID-5 can have data loss
after a system failure.Data loss occursbecause a systemfailure causesthe data andparity
in the RAID-5volume to becomeunsynchronized.Loss of synchronizationoccurs because
the status of writes that were outstanding at the time of the failure cannot be determined.
If a loss of sync occurs while a RAID-5 volume is being accessed, the volume is described
as having stale parity. The parity must then be reconstructed by reading all the non-parity
columns within each stripe, recalculating the parity, and writing out the parity stripe unit
in thestripe. Thismust be done for every stripe in thevolume, soit can take a long time to
complete.
Caution While the resynchronization of a RAID-5 volume without log plexes is being
performed, any failure of a disk within the volume causes its data to be lost.
Besides the vulnerability to failure, the resynchronization process can tax the system
resources and slow down system operation.
RAID-5 logs reduce the damage that can be caused by system failures, because they
maintain a copy of the data being written at the time of the failure. The process of
resynchronization consists of reading that data and parity from the logs and writing it to
the appropriate areas of the RAID-5 volume. This greatly reduces the amount of time
needed for a resynchronization of data and parity. It also means that the volume never
becomes truly stale. The data and parity for all stripes in the volume are known at all
times, so the failure of a single disk cannot result in the loss of the data within the volume.