FUJITSU CentricStor V3.1D - Manual (Page 73)

U41117-J-Z125-7-76 73
Selected system administrator activities Reorganization
4.4 Reorganization
A brief overview of the reorganization of tape cartridges can be found on page 37 .
4.4.1 Why do we need reorganization?
Reorganizations are performed for the following four reasons:
1. Effective use of the physical volumes’ capacity
There are two situations in which logical volumes may be rendered invalid on a physical
volume:
– When removing logical volumes (see section “Logical Volume Operations » Erase
Logical Volumes” on page 242 ), the VLM sends an internal delete command to the
PLM. This causes the PLM to remove the logical volumes from its pool, and flag the
affected areas of the physical volumes in its data maintenance facility (PV file) as
invalid.
– If the host modifies a logical volume, the VLM sends a save request to the PLM. This
causes the PLM to save the new version of the logical volume by appending it to the
same physical volume or a different physical volume. The old version of the logical
volume then becomes invalid.
Over time, the second situation in particular causes a build-up of invalid logical
volumes on a physical volume. If a physical volume contains nothing but invalid
logical volumes, it becomes a scratch tape and can be overwritten.
The purpose of reorganization is to free up any physical volumes with a very low
occupancy level, i.e. to relocate any logical volumes still valid to another physical
volume (write tape).
2. Refreshing the physical volumes
Physical volumes are subject to physical and chemical aging, which means that even
without read and write accesses they can become unusable after a long time. Regular
reorganization of physical volumes which have not been accessed for a long time re-
freshes the magnetization of the tapes and prevents age-related loss of the magnetiza-
tion.
3. Occurrence of a read or write error (faulty status)
Physical volumes on which a read or write error has occurred and which are thus in faul-
ty
status are reorganized so that they can be taken out of service and the logical volu-
mes affected can be backed up again.
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