Veritas InfoScale™ 8.0 Troubleshooting Guide - AIX

Resynchronizing parity on a RAID-5 volume

In most cases, a RAID-5 volume does not have stale parity. Stale parity only occurs after all RAID-5 log plexes for the RAID-5 volume have failed, and then only if there is a system failure. Even if a RAID-5 volume has stale parity, it is usually repaired as part of the volume start process.

If a volume without valid RAID-5 logs is started and the process is killed before the volume is resynchronized, the result is an active volume with stale parity.

The following example is output from the vxprint -ht command for a stale RAID-5 volume:

V   NAME       RVG/VSET/COKSTATE     STATE     LENGTH     READPOL     PREFPLEX   UTYPE
PL  NAME       VOLUME     KSTATE     STATE     LENGTH     LAYOUT      NCOL/WID   MODE
SD  NAME       PLEX       DISK       DISKOFFS  LENGTH     [COL/]OFF   DEVICE     MODE
SV  NAME       PLEX       VOLNAME    NVOLLAYR  LENGTH     [COL/]OFF   AM/NM      MODE
...
v   r5vol      -          ENABLED    NEEDSYNC  204800     RAID        -          raid5
pl  r5vol-01   r5vol      ENABLED    ACTIVE    204800     RAID        3/16       RW
sd  disk01-01  r5vol-01   disk01     0         102400     0/0         hdisk3     ENA
sd  disk02-01  r5vol-01   disk02     0         102400     1/0         hdisk4     dS
sd  disk03-01  r5vol-01   disk03     0         102400     2/0         hdisk5     ENA
...

This output lists the volume state as NEEDSYNC, indicating that the parity needs to be resynchronized. The state could also have been SYNC, indicating that a synchronization was attempted at start time and that a synchronization process should be doing the synchronization. If no such process exists or if the volume is in the NEEDSYNC state, a synchronization can be manually started by using the resync keyword for the vxvol command.

Parity is regenerated by issuing VOL_R5_RESYNC ioctls to the RAID-5 volume. The resynchronization process starts at the beginning of the RAID-5 volume and resynchronizes a region equal to the number of sectors specified by the -o iosize option. If the -o iosize option is not specified, the default maximum I/O size is used. The resync operation then moves onto the next region until the entire length of the RAID-5 volume has been resynchronized.

For larger volumes, parity regeneration can take a long time. It is possible that the system may shut down, or the system may crashe before the operation is completed. In case of a system shutdown, the progress of parity regeneration must be kept across reboots. Otherwise, the process has to start all over again.

To avoid the restart process, parity regeneration is checkpointed. This means that the offset up to which the parity has been regenerated is saved in the configuration database. The -o checkpt=size option controls how often the checkpoint is saved. If the option is not specified, the default checkpoint size is used.

Because saving the checkpoint offset requires a transaction, making the checkpoint size too small can extend the time required to regenerate parity. After a system reboot, a RAID-5 volume that has a checkpoint offset smaller than the volume length starts a parity resynchronization at the checkpoint offset.