HP Veritas Volume Manager 5.0 -UX 11i v3 manual Volume resynchronization, Dirty flags

Understanding Veritas Volume Manager

Volume resynchronization

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Volume resynchronization

When storing data redundantly and using mirrored or RAID-5 volumes, VxVM ensures that all copies of the data match exactly. However, under certain conditions (usually due to complete system failures), some redundant data on a volume can become inconsistent or unsynchronized. The mirrored data is not exactly the same as the original data. Except for normal configuration changes (such as detaching and reattaching a plex), this can only occur when a system crashes while data is being written to a volume.

Data is written to the mirrors of a volume in parallel, as is the data and parity in a RAID-5 volume. If a system crash occurs before all the individual writes complete, it is possible for some writes to complete while others do not. This can result in the data becoming unsynchronized. For mirrored volumes, it can cause two reads from the same region of the volume to return different results, if different mirrors are used to satisfy the read request. In the case of RAID-5 volumes, it can lead to parity corruption and incorrect data reconstruction.

VxVM needs to ensure that all mirrors contain exactly the same data and that the data and parity in RAID-5 volumes agree. This process is called volume resynchronization. For volumes that are part of the disk group that is automatically imported at boot time (usually aliased as the reserved system- wide disk group, bootdg), the resynchronization process takes place when the system reboots.

Not all volumes require resynchronization after a system failure. Volumes that were never written or that were quiescent (that is, had no active I/O) when the system failure occurred could not have had outstanding writes and do not require resynchronization.

Dirty flags

VxVM records when a volume is first written to and marks it as dirty. When a volume is closed by all processes or stopped cleanly by the administrator, and all writes have been completed, VxVM removes the dirty flag for the volume. Only volumes that are marked dirty when the system reboots require resynchronization.

Resynchronization process

The process of resynchronization depends on the type of volume. RAID-5 volumes that contain RAID-5 logs can “replay” those logs. If no logs are available, the volume is placed in reconstruct-recovery mode and all parity is regenerated. For mirrored volumes, resynchronization is done by placing the volume in recovery mode (also called read-writeback recovery mode).