DASD partitions

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Ubuntu 22.04 LTS LPAR mode z/VM guest KVM guest

Partitioning DASDs has the same advantages as for other disk types, but there are some prerequisites and a special tool, fdasd.

A DASD partition is a contiguous set of DASD blocks that is treated by Linux® as an independent disk and by the traditional mainframe operating systems as a data set.

With the Linux disk layout (LDL) and the CMS disk layout, you always have a single partition only. This partition is defined by the LDL or CMS formatted area of the disk. With the compatible disk layout, you can have up to three partitions.

There are several reasons why you might want to have multiple partitions on a DASD, for example:

Limit data growth
Runaway processes or undisciplined users can consume disk space to an extend that the operating system runs short of space for essential operations. Partitions can help to isolate the space that is available to particular processes.
Encapsulate your data
If a file system gets damaged, this damage is likely to be restricted to a single partition. Partitioning can reduce the scope of data damage.

Recommendations

  • Use fdasd to create or alter partitions on ECKD™ type DASDs that are formatted with the compatible disk layout. If you use another partition editor, it is your responsibility to ensure that partitions do not overlap. If they do, data damage occurs.
  • Leave no gaps between adjacent partitions to avoid wasting space. Gaps are not reported as errors, and can be reclaimed only by deleting and re-creating one or more of the surrounding partitions and rebuilding the file system on them.

A disk need not be partitioned completely. You can begin by creating only one or two partitions at the start of your disk and convert the remaining space to a partition later.

There is no facility for moving, enlarging, or reducing partitions, because fdasd has no control over the file system on the partition. You can only delete and re-create them. Changing the partition table results in loss of data in all altered partitions. It is up to you to preserve the data by copying it to another medium.