Seagate Turbo SSHD Architecture
The enterprise space has seen numerous caching and tiering approaches spring up to help marry the performance of SSDs with the capacity of HDD's. Unfortunately, these approaches typically involve the purchase of expensive software and/or hardware appliances, which incur increased costs for administration of the caching/tiering solution.
Seagate's SSHD cache management is entirely transparent to the user. Seagate's proprietary AMT (Adaptive Memory Technology) algorithms intelligently identify hot data at the block level, assess the data, and acts accordingly. Caching at the block level, as opposed to caching files, is important. In many cases, the same data blocks can accelerate several applications simultaneously.
Hot data is placed into NAND cache, with a copy retained on the platters to insure data integrity in the event of NAND failure. The AMT algorithm is adaptive; it constantly studies the changing hot data patterns and then evicts and promotes new hot data to the cache based upon access times from the platter. Data promotion is handled judiciously to reduce undue wear on the NAND, ensuring a long lifecycle. The drive avoids caching sequential data, which plays to the strength of the HDD and reduces wear on the NAND, only caching the hardest-to-read random data. Writing random data sequentially to the cache also helps extend the longevity of the NAND. The use of eMLC with 15K P/E Cycles, in lieu of standard consumer-grade NAND with 3K P/E cycles, also ensures plenty of endurance over the warranty period.
Caching random read data has numerous other positive side effects. Alleviating much of the head movement during intense mixed read/write workloads boosts random write performance and sequential access latency. One would also assume that the use of caching would lead to less wear and tear on the head units and actuators, though Seagate has guaranteed the drive with standard AFR and MTBF specifications.
The NAND cache of the SSHD is also multi-segmented, with a small 8MB layer of SLC designated as NVC (Non-Volatile Cache). While the cached data in the eMLC is lost when the SSHD is powered down (not a concern with 24/7 duty cycles in the enterprise), the data in the SLC layer is held for up to 90 days when powered down. The NVC layer is also used to save data stored in DRAM in the event of an unsafe power loss, creating an extra layer of power-loss protection not provided by standard HDD's.
Seagate Turbo SSHD Specifications
The SSHD uses typical perpendicular recording technology and comes in several versions with both 4K native and 5xx emulation. There is further segmentation depending upon the level of encryption required: Standard, Secure Encryption, and FIPS 140-2 with encryption models are available.
Cache comes in three tiers, with 128MB of multi-segmented DRAM, 32GB of eMLC and 8MB of NVC on every model. Power consumption is unchanged at 8.5 Watts during typical operation, and 5.9 Watts at Idle.
The Turbo SSHD features a Nonrecoverable Read Error rate of 1 per 10E16, and an AFR of 0.44%. The drive also features an enterprise-standard five-year limited warranty.
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