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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.
If you are restoring some of the files in the file system, you should first remove the data-full Storage Checkpoints that are no longer needed. If you have very limited free space on the file system, you may have to remove all data-full Storage Checkpoints in order for the restore to succeed.
To avoid unnecessary Storage Checkpoint removal, instead of using a low quota limit use the SFDB utility to set up a Monitoring Agent to monitor file system space usage. When file system space usage exceeds a preset threshold value (say, 95 percent full), the Monitoring Agent alerts the system administrator and optionally grows the volume and the file system. Automatic notifications to the system administrator on the status of space usage and file system resizing are available through electronic mail, the syslogd(1M) program, or by logging messages to a simple log file.
Always reserve free disk space for growing volumes and file systems. You can also preallocate sufficient space for each file system when the file system is first created or manually grow the file system and logical volume where the file system resides.
See the vxassist(1) and fsadm_vxfs(1) manual pages for more information.