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A hybrid drive, hybrid hard drive (HHD), hybrid hard disk drive (H-HDD), or solid state hybrid drive (SSHD) is a type of large-buffer computer hard disk drive. It is different from standard hard drives in that it integrates a cache using non-volatile memory or even a small solid-state drive (SSD or SSH drive). Although the cache typically uses non-volatile flash memory, some drives use battery-backed volatile RAM (a hybrid RAM disk). The flash memory buffer can speed up repeated reads from the same location; a RAM buffer speeds both reads and writes, but must be written to backup storage before power is lost.
In 2007 Seagate and Samsung both introduced hybrid HDDs primarily targeted to OEM notebook computer manufacturers. Although the technology was initially supported by Microsoft, a number of issues prevented their success over the next year in the market with Windows Vista.
In May 2010, Seagate launched the Momentus XT 7200 RPM 2.5-inch solid state hybrid drive, which has a 4 GB single-level cell NAND chip, used as a read cache managed by the drive itself. Seagate calls this adaptive memory and claims it removes the OS, driver, and software dependency previously required to take advantage of the integrated flash memory. The SSD portion of this new drive is also now a larger 4 GB, compared to 256 MB in the past. Benchmark results conducted and published by Seagate of the Momentus XT place the 7200 RPM hybrid drive's performance firmly between high-end 10,000 RPM drives and solid-state drives on a number of tasks, such as system boot time and application launch speed. For example, according to these benchmarks, system boot time using this hybrid drive is approximately 41% faster than the same task using a non-hybrid 7200 RPM drive, while a "best-in-class" solid-state drive performs approximately 44% faster than a standard 7200 RPM drive on the same test. According to The Register, in the first year after launch Seagate sold 350,000 units of this hybrid drive.
Unlike most standard hard drives, the hybrid drive in its normal state has its platters at rest, with the motor not consuming power or generating heat. Any data written to the hybrid drive is written to the buffer; there is no risk of data loss in the event of power failure as the buffer's flash memory is not volatile. When reading data from the platters extra data is read and stored in buffer memory (which need not be non-volatile flash memory) in the hope of anticipating future requirements as in any disk cache. Data required for the next boot-up can be stored in the non-volatile buffer before shutting down the computer.
The hybrid drive's platters will spin up to move data from the write buffer to the platters when nearly full, and to read data which is not in the cache.
Early estimates place the actual hard drive usage (when the platters are spinning) at anywhere between 1.25% and 10% for typical single users, although there are obviously situations where platter usage will be much higher, such as the encoding or editing of very large video files, producing reports from a database, etc.
As of early 2008 the special features of hybrid drives were taken advantage of only by Microsoft's Windows Vista and Windows 7 operating systems. The files required for booting are stored in the fast flash memory, reducing boot time by about 11%. Microsoft uses the name ReadyDrive to describe the software side of this technology. While hybrid drives are not required for Vista Premium certification of laptops, some confusion arose as to whether such drives would be mandatory.
The hybrid drive command interface will be standardized in the new Revision 8 of the ATA standard. The ATA-8 draft provides a new set of commands allowing the host to manage the on-board flash memory or other types of non-volatile (NV) cache, for example pinning some LBAs in the NV cache.
In 2010 Seagate released the Momentus XT, which uses so-called "adaptive memory" for its SSD portion, which is not reliant on driver support from the operating system. This removes the need for a special operating system, and the speed benefits can be used by any OS.
Claimed benefits 
Manufacturers claim several benefits of the hybrid drive over standard hard drives, especially for use in notebook computers. Samsung claims the chief benefits are speed of data access and consequent faster computer boot process, decreased power consumption, and improved reliability.
Inherent drawbacks 
- Lower performance (access time) for non-cached data. If the data being accessed is not in the cache and the drive has spun down, access time will be greatly increased since the platters will need to spin up.
- Lower performance for small disk writes. Flash memory is significantly slower for writing small data, an effect that is amplified by use of journaling file systems.[dubious ]
- Slightly increased cost. Hybrid hard drives are currently slightly more expensive than their non-hybrid counterparts, because of the higher cost of flash memory.
- Reduced lifetime. A hard drive, once spinning, suffers almost no wear. A significant proportion of wear arises during the spin-up and spin-down processes. Indeed, the number of spin-ups is often given as the indication of the lifetime of a hard drive. Also, flash memory allows far fewer write cycles than a hard disk.
- Increased perceived noise production. A hybrid hard drive, spinning up and down, may make drive noise more noticeable, especially with varying usage conditions (i.e., fans and hard drive spinning up on usage).
- Increased power usage with spin-ups. A hybrid drive requires spin-up and spin-down more often than a normal hard drive, which is often spinning constantly. Hard drives also draw more power during spin-up.
- Lower recoverability. Hybrid hard drives, based on storage to both a flash component and a hard drive component, and bound to use by a specific OS (i.e., Windows Vista). Data recovery often requires specialized data recovery services which are expensive, and sometimes the data stored within the NAND flash may be unrecoverable.
Software implementation 
Hybrid drives may also be implemented in software, using other memory instead of a buffer built into the hard drive. Examples of such systems include the SuperCache caching software (implements hybrid RAM drive), and Windows ReadyBoost and ReadyDrive, which use separate flash memory (removable and non-removable, respectively) and hard disk to make a hybrid drive. Performance of the combined drive may be affected by the speed of the CPU and speed of the separate memory used, in addition to the speed of the hard drive interface.
The primary benefits of a hybrid drive may be created from a separate HDD and flash memory device, managed by software, such as ReadyDrive in Windows. A similar technology has been implemented by Diskeeper Corporation's ExpressCache software in Samsung laptops.
Windows Vista and Windows 7 natively support the use of hybrid drives (ReadyDrive).
In fall 2011, OCZ Technology began selling versions of their Synapse Cache SSD. The drives are overprovisioned to provide 50% of their rated capacity, alongside NVELO Dataplex software caching, to accelerate existing drives.
Hybrid RAM disk 
The hybrid RAM disk uses a large buffer of volatile SDRAM memory with battery backup instead of flash memory to cache hard disk data during normal use. The buffer reduces time and energy spent accessing the hard disk. If the RAM cache needs to be filled at start-up, hybrid RAM disks may not speed the start-up process as much as flash-based hybrid disks.
Battery backup allows data to be written to permanent storage when the device is shut down, and may allow data retention in RAM for limited periods.
Frequently accessed data is stored in DRAM, while all the data is stored in hard disk. As a result, the hard disk needs to be accessed only when the processor requests data that is not frequently used.
Hybrid RAM disks are claimed to offer significantly faster read-write speeds compared to standard hard disks.
Some RAM based SSDs may be used in combination with a hard disk to create a hybrid RAM disk.
When data is written to memory, the corresponding block in memory is marked as dirty, and all dirty blocks can be flushed to the actual hard drive based on the following criteria: Time (e.g., every 10 seconds, flush all dirty data); Threshold (when the ratio of dirty data to SSD size exceeds some predetermined value, flush the dirty data); Loss of power/computer shutdown.
See also 
- Disk storage
- Solid-state drive
- Intel Turbo Memory
- Smart Response Technology (Intel SSD Caching)
- Windows Vista I/O technologies
- Non-volatile memory
- Fusion Drive
- Schmid, Patrick (2007-07-13). "Should You Care About Hybrid Hard Drives?". Retrieved 2010-06-06.
- Hetzler, Steven R. (2009-02). "The Storage Chasm: Implications for the Future of HDD and Solid State Storage". IBM. Archived from the original on 2011-07-25. Retrieved 2012-08-28.
- Coughlin, Thomas (2010-03-24). "Does it make sense to combine HDDs and flash memory?". GLG Group. Retrieved 2010-06-06.
- Maleval, Jean-Jacques (2010-03-10). "Comeback of Seagate in Hybrid HDD?". Storage Newsletter. Retrieved 2010-06-06.
- Ricker, Thomas (2007-03-07). "Samsung's Hybrid Hard Drive (HHD) released to OEMs". Engadget. Retrieved 2007-04-25.
- "Seagate's Momentus XT Reviewed, Finally a Good Hybrid HDD". AnandTech. Retrieved 2012-11-30.
- Kirsch, Nathan (2010-05-24). "Seagate Momentus XT 500GB Solid State Hybrid Drive Review". Legit Reviews. Retrieved 2010-06-06.
- "Momentus XT 7200 RPM Seagate". Seagate. Retrieved 2010-10-10.
- Intel trying a flash cache again, The Register
- "Hitachi-LG Data Storage". Hlds.co.kr. Retrieved 2012-11-30.
- "iMac - Performance". Apple. Retrieved 2012-11-30.
- "Hybrid drive to extend notebook battery life", 26.04.2005, By Kanellos, Michael, CNET News.com
- "Notebook Storage Gets Buffer" Maximum PC July 2005, pg. 15
- Microsoft Windows Vista ReadyBoost and ReadyDrive Primer, dvnation.com
- what are hybrid hard drives (HHDs)?, 2007/10/08, by Paul Godden, skymonkey.org
- Fischer, Ken (2006-06-14). "Hybrid hard drives are not a Vista Premium requirement". One Microsoft Way (Ars Technica). Retrieved 2008-02-16.
- "Samsung says flash drive boosts speed of Windows Vista". Electronics Weekly. 26 July 2006. Retrieved 2006-01-21.
- Hybrid drives: not so fast!, By Robin Harris, August 27, 2007, ZDNet
- A. Birrell, M. Isard, C. Thacker, T. Wobber. A Design for High-Performance Flash Disks In Operating Systems Review, 41(2), April 2007, pages 88-93.
- Continuous Power vs. Peak Power at Spin-Up, Posted April 17th, 2001 by Charles Kozierok, The PC Guide
- "Samsung Series 7 GAMER to use Diskeeper's ExpressCache". AnandTech. Retrieved 2012-11-30.
- "OCZ Synapse Cache SATA 3 64GB SSD Review - Top Caching Solution At a Great Price". The SSD Review. 2011-11-06. Retrieved 2012-11-30.
- "Write amplification - Wikipedia, the free encyclopedia". En.wikipedia.org. Retrieved 2012-11-30.
- Platinum HDD - combining the harddrive with a DRAM, 2007/08/29, By Andreas Galistel, NordicHardware