Reliance Fault Tolerant File System (Datalight Inc)
Fail Safe File System
Reliance™ is a file system that was developed specifically for embedded systems. Engineers who choose Reliance do so because they cannot take a chance that data could be lost due to file system corruption.
Engineers use Reliance because
- The file system is highly reliable
- The file system architecture is fast and efficient
- The file system enables fast boot
- The file system is ported to VxWorks 5.4, 5.5 and 6.0
Unlike FAT-based file systems, Reliance offers developers the use of transaction points to ensure that directory information and user data are committed to media at regular intervals. Additionally, Reliance never overwrites needed on-media file structures or data. When a power glitch occurs, Reliance simply “rolls back” to the last known good state ensuring that a Reliance disk is always consistent. Embedded devices that use Reliance are trustworthy and able to quickly boot.
Reliance is Highly Reliable
- Keeps track of last known good state of both the file system’s metadata and user data
- Updates information to memory at user-determined transaction points
- Transaction-based approach offers the user an "always consistent" file system
Reliance Architecture is Fast and Efficient
- Saved data is preserved until the next transaction point occurs
- At the time of a transaction point, new data is written to a new area on the storage media
- Transaction is atomic; if a power interruption occurs, the file system will still have last saved data
- Unlike journaling or logging file systems, there is no ongoing recording of data – making Reliance more efficient
Reliance Enables Fast Boot
- There is no need for a chkdsk utility or to check the integrity of the file system at boot time – allowing Reliance to offer a faster boot time compared to FAT-based file systems
After building an image, a developer configures Reliance to work with specific block devices such as resident or removable flash memory, DiskonChip, hard drives or RAM drives. Other Reliance configuration parameters include the disk partition size, block size, file size and transaction point settings.
How Reliance Works
The reliability of Datalight Reliance is based on the preservation of original data until the writing of new data is performed and the file transaction completed. During the file transaction all file system modifications -- including changes to directories, files and metadata -- are stored on an area of the media that is currently unused and does not contain "committed" data. The possibility of corruption is eliminated because the data of the new transaction is written to an unused portion of the media. Writing to an unused portion of the media allows the previous state of the file system to be untouched. Valid data from the previous transaction is always available.
A significant event in the operation of the Reliance file system is the setting of a transaction point. When one or more file operations are successfully completed and data is committed to the storage media, a transaction point is set by the file system. It is at that moment that the file system changes from the previous state to the new state. The transaction point defines the state of the file system at the completion of one or more file I/O operations and remains the valid transaction point until the next transaction point is established.
How Reliance Preserves Data
Reliance keeps track of unused (or free) data blocks on the storage media and always writes data to these blocks, thereby preserving all existing valid and committed (to storage) file data. Under no circumstances does Reliance overwrite existing data blocks. In this manner, the previous state of the file system remains intact on the storage media during the current write operation.
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