Estimate Use of Disk Space
18 DD400 Series Restorer User Guide
Estimate Use of Disk Space
The restorer’s use of compression when storing data means that you can look at the use of disk
space in two ways: physical and virtual. (See
“Data Compression” on page 3 for details about
compression.) Physical space is the actual disk space used on the restorer. Virtual space is the
amount of space needed if all data and multiple backup images were uncompressed.
• Through the restorer, the filesys show space command (or the alias of df) shows both
physical and virtual space. See
“Manage File System Use of Disk Space” on page 19.
• Directly from clients that mount a restorer, use your usual tools for displaying a file system’s
physical use of space.
The restorer generates log messages as the file system approaches its maximum size. The following
information about data compression gives guidelines for disk use over time.
The amount of disk space used over time by a restorer depends on:
• The size of the initial full backup.
• The number of additional backups (incremental and full) over time.
• The rate of growth for data in the backups.
For data sets with average rates of change and growth, data compression generally matches the
following guidelines:
• For the first full backup to a restorer, the compression factor is about 3:1. Disk space used on
the restorer is about one-third the size of the data before the backup.
• Each incremental backup to the initial full backup has a compression factor of about 6:1.
• The next full backup has a compression factor of about 60:1. All data that was new or changed
in the incremental backups is already in storage.
• Over time, with a schedule of weekly full and daily incremental backups, the aggregate
compression factor for all the data is about 20:1. The compression factor is lower for
incremental-only data or for backups without much duplicate data. Compression is higher with
only full backups.
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