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The LASS ("Large Allocation Storage System") is a storage system for digital video which divides the disk into 10,000 "large allocations" whose size is dependent on the disk size (typically 20MB to 100MB each). It is optimised for continuously recording CCTV and megapixel video recording.
This provides for efficient allocation with a very small central table, stored in a single block, and stored in three locations over the disk for redundancy. When the assignments of any of the Large allocation units ("LAUs") change, this table is updated. Because the table is so small, it is memory resident and checked each time it is modified to ensure it is consistent, and the entire table written to disk atomically. The term "atomically" is used to mean that the write logically succeeds or fails in entirety (it may be more than one sector).
When restarting after power down (even unexpected power down) all three LAU tables are scanned and the newest which is valid is used (partially written LAU tables fail their checksum). This takes only a few milliseconds. New LAUs are allocated for each live channel, thus ensuring no additional free-list information needs to be stored or regenerated.
This provides for rapid restart (only a few milliseconds) with no risk of corruption or data loss.
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