Sun Microsystems 3.2 manual Safeguarding the VTOC Information

Safeguarding the VTOC Information

Caution – When creating shadow volume sets, do not create shadow or bitmap volumes using partitions that include cylinder 0. Data loss might occur.

The Solaris system administrator must be knowledgable about the virtual table of contents (VTOC) that is created on raw devices by the Solaris operating system. The creation and updating of a physical disk’s VTOC is a standard function of the Solaris operating system. Software applications like the Sun StorEdge Availability Suite, the growth of storage virtualization, and the appearance of SAN-based controllers have made it easy for an uninformed Solaris system administrator to allow a VTOC to be altered inadvertently. Altering the VTOC increases the possibility of data loss.

Remember these points about the VTOC:

■A VTOC is a software-generated virtual table of contents based on the geometry of a device and written to the first cylinder of that device by the Solaris format(1M) utility.

■Various software components such as dd(1M), backup utilities, point-in-time copy software, and remote mirror software can copy the VTOC of one volume to another volume if that volume includes cylinder 0 in its mapping.

■If the VTOC of the source and destination volumes are not identical, some type of data loss might occur. This data loss might not be detected initially, but can be detected later when other utilities are used, like fsck(1M), or when the system is rebooted.

When first configuring and validating volume replication, save copies of all affected devices’ VTOCs using the prtvtoc(1M) utility. The fmthard(1M) utility can be used to restore them later, if necessary.

■When using volume managers like SVM and VxVM, copying between individual volumes created under these volume managers is safe. VTOC issues are avoided because the VTOC is excluded from volumes created by these volume managers.

■When formatting individual partitions on a raw device, for all partitions except the backup partition, make sure the partitions do not map cylinder 0, which contains the VTOC. When using raw partitions as volumes, you are the volume manager and you need to exclude the VTOC from partitions that you configure.

■When formatting the backup partition of a raw device, make sure that the physical geometries of the source and destination devices are identical. (Partition 2, by default, maps all cylinders under the backup partition.) If identical device sizing is not possible, make sure that the source backup partition is smaller than the destination partition and that the destination partition does not map cylinder 0.

Chapter 1 Point-in-Time Copy Software Troubleshooting Tips 3