VERITAS Storage Foundation 4.1 Oracle Administrator's Guide

Chapter 8, Using Storage Checkpoints and Storage Rollback
Prerelease 8 September 2005, 8:55am Determining Space Requirements for Storage Checkpoints
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Determining Space Requirements for Storage Checkpoints
To support Block-level Incremental (BLI) Backup and storage rollback, the file systems need extra
disk space to store the Storage Checkpoints. The extra space needed depends on how the Storage
Checkpoints are used. Storage Checkpoints that are used to keep track of the block changes contain
only file system block maps, and therefore require very little additional space (less than 1 percent of
the file system size).
When you use VERITAS NetBackup to back up your database, VERITAS NetBackup creates one
set of Storage Checkpoints to provide a consistent view of the file systems for the database
backups. The space required to hold this additional set of Storage Checkpoints depends on how
busy the database load is when the backup is running. If the database is offline during the entire
backup window, there is no additional space required.
If the database is online while the backup is running, the additional space required by each file
system for Storage Checkpoints depends on the duration of the backup and the database workload.
If workload is light during the backup or the backup window is relatively short (for example, for
incremental backups), for most database configurations, an additional 10 percent of the file system
size will be sufficient. If the database has a busy workload while a full backup is running, the file
systems may require more space.
To support Storage Checkpoints and storage rollback, VxFS needs to keep track of the original
block contents when the Storage Checkpoints were created. The additional space needed is
proportional to the number of blocks that have been changed since a Storage Checkpoint was taken.
The number of blocks changed may not be identical to the number of changes. For example, if a
data block has been changed many times, only the first change requires a new block to be allocated
to store the original block content. Subsequent changes to the same block require no overhead or
block allocation.
If a file system that has Storage Checkpoints runs out of space, by default VxFS removes the oldest
Storage Checkpoint automatically instead of returning an ENOSPC error code (UNIX errno 28-
No space left on device), which can cause the Oracle instance to fail. Removing Storage
Checkpoints automatically ensures the expected I/O semantics, but at the same time, eliminates a
key recovery mechanism.
When restoring a file system that has data-full Storage Checkpoints from tape or other offline
media, you need extra free space on the file system. The extra space is needed to accommodate the
copy-on-write algorithm needed for preserving the consistent image of the Storage Checkpoints.
The amount of free space required depends on the size of the restore and the number of Storage
Checkpoints on the file system.
If you are restoring the entire file system, in most cases, you no longer need the existing Storage
Checkpoint. You can simply re-make the file system using the mkfs command, and then restore
the file system from tape or other offline media.