Seagate Hybrid Laptop Thin SSHD 500GB Review

Seagate has introduced the next generation of Solid State Hybrid Drives, commonly referred to as "SSHD." These drives use a small amount of MLC NAND to accelerate the performance of a 5400 RPM spinning disk. Today we test the mobile version against other available SSD caching solutions.


Time To Ready

The architectural improvements with this generation of hybrid drives has allowed Seagate to go from 7200 RPM to 5400 RPM HDDs. This is due to increased optimization of the Adaptive Memory Technology (AMT) that identifies and caches hot data.

The Ultrabook and Windows 8 specifications have called for drives with radically lower power consumption, facilitated by lower RPM HDDs. The double edged aspect is that these new specifications also require enhanced performance in certain states, specifically performance during boot and "time to ready."

"Time to ready" is the amount of time the HDD requires to begin pumping data to the OS during boot, or after resumption from a sleep or hibernate state. HDDs require time to spin up, then these have to find the data on the platter and begin sending it back to the OS. This makes HDDs notoriously slow during boot and resumption. Seagate has found a method to boost performance during these tasks, accomplished by merely keeping all data required during the first minute of HDD operation in the cache. During boot and resumption tasks the NAND provides all data to the OS while the platter spins up in the background.

Most data is cached as blocks depending upon how frequently it is requested. In order to provide a time-to-ready speed under 4 seconds, as required by the new tighter requirements, AMT has a list of files and blocks that are required to meet these performance objectives. This data is 'pinned' into cache regardless of how often it is requested. This provides very fast bootup and resumption speeds in the latest Ultrabooks and mobile devices.

Pinning is an interesting feature, and we would like to see the ability for users to pin their own data in the cache in the future. There are plans for SSHDs with up to 32GB of NAND in future product generations, and in conjunction with pinning this would allow users direct control over boosting certain applications and files. For the time being the limited amount of NAND will not allow for user controlled pinning.

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Cache Tiering

All SSHD models hold 8GB of MLC NAND, the same capacity featured on the previous generation Momentus XT. SSHDs feature MLC instead of the SLC utilized in the previous generation. Seagate has also doubled the volatile DRAM cache from 32MB to 64MB of cache. This doubling of cache allows the drive to "catch" more data in this upper caching tier. This area allows the drive to cache write data quickly before it is passed down to the NAND. This method of passing data from one cache layer to another is referred to as "tiering."

Things get interesting when the data is passed from DRAM down to NAND in this case. The switch from SLC to MLC in the new generation required Seagate to design a method of increasing the endurance of the MLC NAND. This is accomplished by adding a second tier of cache, and surprisingly that layer is SLC NAND. This method is very similar to approaches we have recently observed in other SSDs, where a portion of the MLC NAND is actually programmed to act as SLC. From a controller and firmware standpoint this is actually simple; merely programming one bit per cell in a section of the existing MLC effectively transforms this portion of the MLC into SLC.

This provides a fast caching area for write data, and depending upon the situation the firmware will either pass the data down to the MLC, or the disk. This creates 4 tiers of storage, with the first being DRAM, then SLC, MLC, and then the platter. This allows a portion of the NAND (Seagate is not disclosing how much) to be used for endurance-killing write data. The 100,000 P/E cycles of SLC will bear this heavy workload more effectively than the 3,500-5,000 cycles provided by MLC.

Another benefit of multiple tiers is write combining. These high endurance scratch areas can convert random write data into sequential data before passing this data down to the lower endurance NAND. Random data creates much more wear on NAND, and converting random writes into sequential writes increases the endurance of the underlying MLC NAND.

Intelligently assessing the data, and caching small amounts, also improves the longevity of the NAND. The AMT engine caches blocks of data (not files) that are commonly requested. If the data is sequential in nature, which HDDs excel at, the blocks will not always be placed in the cache. The engine decides upon the nature of the data and the amount of access time required to pass the data from the HDD into its process of determining whether or not to cache it in NAND. Caching only blocks allows AMT the ability to cache very small bits of data that can provide big performance gains, maximizing the small amount of cache.

The caching algorithms are adaptive, and if the users’ habits change significantly AMT will evict older less used data in exchange for more "popular" data. Adaptive caching algorithms will change to suit the users’ needs over a period of time.

Write data is always committed to the platters of the HDD, ensuring that only a copy of the data is held in the NAND. This ensures the safety of the data in the event of NAND failure.

Availability and Warranty

The Laptop Thin SSHD is available at 500GB, and the Laptop SSHD is available in 1TB.

Both SSHDs feature a three year warranty.


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