Veritas Storage Foundation™ for Oracle 5.0.1 Administrator's Guide

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.
Using Storage Checkpoints and Storage Rollback
Space requirements for Storage Checkpoints
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