Next: WAFL Image Dump Up: Logical vs. Physical File Previous: Logical BackupIn its simplest form, physical backup is the movement of all data fromone raw device to another; in the context of file system backup the sourcedevices are disks and the destination devices may include disk, CD-Rom,floppy, Zip drives, and of course, tape.
It is a straightforward extension to the simple physical backup described inthe preceding paragraph to interpret the file system meta-data sufficientlyto determine what disk blocks are in use and only back those up. All filesystems must have some way of determining which blocks are free, and thebackup procedure can interpret that information to only back up the blocksthat are in use. Naturally, this requires that the block address of eachblock written to the backup medium be recorded so that restore can put thedata back where it belongs.
The primary benefits of a physical backup scheme are simplicity andspeed. It is simple because every bit from the source device is copied tothe destination; the format of the data is irrelevant to the backupprocedure. It is fast because it is able to order the accesses to the mediain whatever way is most efficient.There are a number of limitations to physical backup, however. First, sincethe data is not interpreted when it is written, it is extremelynon-portable; the backup data can only be used to recreate a filesystem if the layout of the file system on disk has notchanged since the backup was taken. Depending on the file systemorganization, it may even be necessary to restore the file system to disksthat are the same size and configuration as the originals.Second, restoring a subset of the filesystem (for example, a single file which was accidently deleted) is not verypractical. The entire file system must be recreated before the individualdisk blocks that make up the file being requested can be identified.Third, the file system must not be changing when the backup is performed,otherwise the collection of disk blocks that are written to disk will likelynot be internally consistent.Finally, the coarse grained nature behind this method leads to itsown difficulties. Because file system information is not interpreted bythe backup procedure, neither incremental backups norbacking up less than entire devices is possible.A raw device backupis analogous to a fire hose. Data flows from the device simply and rapidly-- but it is all the you can do to hold the hose. Finer grained control isgenerally impossible.
These negative aspects of physical backup have until now caused it to havevery limited application.Physical backup is used in a tool from BEICorporation thataddresses the problem of restoring NTsystems after catastrophic failure of the system disk [Edw97]. Twolarge experiments at getting Terabyte per hour backup performance mentionthe use of raw (or device) backup and contain performance measures thatvalidate the intuition that it obtains extremely good performance[CCC98,Sun97].
Next: WAFL Image Dump Up: Logical vs. Physical File Previous: Logical BackupLogical vs. Physical File System Backup
OSDI '99
The article you've shared discusses the concepts of logical and physical file system backups, primarily focusing on the details, benefits, and limitations of physical backups. Let's break down the concepts used in the article:
Physical Backup:
- Definition: Movement of all data from one raw device to another.
- Context: In file system backup, source devices are disks, and destination devices can be various media such as disks, CD-ROMs, floppies, Zip drives, or tapes.
- Process: Involves copying every bit from the source device to the destination, disregarding the format of the data.
- Speed & Simplicity: Benefits include simplicity due to copying all data and speed due to efficient access ordering.
Limitations of Physical Backup:
- Non-Portability: The backup data can only recreate a file system if the layout hasn't changed since the backup.
- Restoration Challenges: Difficulty in restoring a subset of the file system (e.g., a single file) as the entire file system must be recreated first.
- Data Consistency: Backup inconsistency if the file system changes during the backup process.
- Lack of Granularity: Inability to perform incremental backups or back up less than entire devices due to the coarse-grained nature.
Analogy:
- Raw Device Backup: Compared to a fire hose - data flows rapidly but lacks finer control.
Specific References:
- BEI Corporation Tool: Utilizes physical backup to restore NT systems post catastrophic disk failures.
- Performance Validation: Large experiments confirming high backup performance when using raw or device backup methods.
WAFL Image Dump:
- Referenced as the title of the article.
The article delves into the distinctions between logical and physical backups, emphasizing the advantages and drawbacks of employing physical backup methods in file system management and data recovery.
