Data Retention

Exchanges traditionally rely on magnetic tape and optical disk media for data storage. Both of these provide good data retention features when used within specifications and with good quality media. Unfortunately the tape and optical equipment is now quite old, having exceeded its expected lifetime. Reconditioning these units is possible in some cases, but this is expensive and difficult. Worse, the media itself is now difficult to obtain, such that the entire solution is compromised. This is the key reason why the DDS has proven such as major benefit to Telecos: it removes the reliance the switches have on these out-dated storage solutions.

However, it is critical that the data retention capabilities of the new solution are at least as good as the original magnetic solution was when it was installed. Here the DDS's design heritage stands up very well, with multiple levels of redundancy and error correction.

DDS uses industrial-grade SLC (Single Level Cell) flash memory which is rated for 100,000 write/erase cycles. Once written, data retention time is rated at 10 years for the raw flash memory. However, both of these parameters are extended in the DDS through advanced techniques.

Standard flash solutions rely on a small and simple embedded hardware controller which decides where to write new data, and where existing data is located. It maps disk addresses to NAND flash pages through its own algorithm. It tries to ensure that NAND blocks are not written repeatedly too many times, since the limit for any block is around 100,000 cycles. With DDS we ensure that the NAND chips are only ever written sequentially, from start to finish, using a highly optimised filesystem specifically developed for NAND chips.

While good design is essential, DDS data retention capability is also proven in the field. Hundreds of DDS units have been installed for more then 6 years without accumulating significant block failures, even using flash technology available at the time. This flash implementation has stood the test of time.

Unique to the DDS, when a NAND flash block returns a bad bit, the DDS detects this right at the top level and swaps to obtain this block from the other chip. In practice this extends the lifetime of the block to the better of the two NAND chips, and certainly protects against early failure which can be expected even quite a distance below the MTBF (mean time before failure).