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SSD/ENT: Read-Optimized Mirrored RAID Using Mixed Drive Classes Disclosure Number: IPCOM000192666D
Original Publication Date: 2010-Jan-28
Included in the Prior Art Database: 2010-Jan-28
Document File: 3 page(s) / 52K

Publishing Venue



Solid state drives are much more expensive than enterprise hard disk drives. Today, users of enterprise storage systems must choose between high cost SSD drives capable of fast I/O, or slower I/O at significantly reduced cost. Customers also have to weigh having a more expensive RAID, such as RAID 10, which offers added data protection versus a cheaper RAID, such as RAID 5, with less data protection. Some applications have a high read-to-write ratio. It is desirable to have the same fast read access as SSD without having to pay the full price of SSD. It is also desirable to have the reliability benefits of a mirrored RAID such as RAID 10 at a reduced cost.

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SSD/ENT: Read-Optimized Mirrored RAID Using Mixed Drive Classes

This invention describes a method of utilizing fast/expensive storage media with slow/inexpensive storage media within a mirrored array.

Physical Configuration

In this invention, more than one type of drive class is used within a RAID array. In the preferred implementation, half of the drives are of one class and the other half are of the other drive class.

There must be at least two drives in the array and at least two drive classes represented. An example preferred implementation referred to later uses 7 Solid State Drives (SSD) and 7 Enterprise Class Drives (ENT).

Alternative physical configurations can have a disproportionate number of drive classes represented as well as more than two drive classes. For example, having 8 SSD drives, 2 SATA drives, and 4 ENT drives. One SSD drive would be in the "slow" side with the 4 ENTs and 2 SATAs. The other 7 SSD drives would be on the "fast" side. Another example is having 5 ENT drives and 3 SSD drives. Four ENT drives would be in the "slow" side and one in the "fast" side together with the 3 SSDs. Such disproportionate configurations would have "slow spots", or areas where data allocated in certain ways would have lower performance than other data. The preferred implementation, however, would attempt to keep the number of fast vs. slow drives equal to avoid the need to have the customer manage where the data is placed in the array in order to get best performance for critical data.

Allocation Algorithm

A mirroring RAID array is used with one side of the mirror having drives that are faster than the other side. (For example, half SSD drives all in one side and the other half ENT in the other side.)

The following diagram shows a Read-Optimized RAID 10 array with 7 SSD drives and 7 Enterprise (ENT).

Each data extent resides on two drives, one in the fast group and one in the slow group. The Read-Optimized RAID 10 array implementation puts each extent (for example A1) on both a SSD drive and a ENT drive.

Alternative mirroring RAIDs such as RAID0, RAID5+1 or RAID10 that rotates data are allowed, but the above implementation is the standard and our preferred implementation.

Data Destage Algorithm

When data is written, it is stripped on both the fast drives and the slow drives. Since data is written to both groups of drives, the performance of writing data is constrained by the slowest drive.


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For the preferred solution of ENT and SSD drives, the ENT drives have a indeterminate seek time. A write to a normal RAID 10 implementation of all ENT drives has the speed of the slowest of its two writes. In the Read-Optimized RAID 10 implementation on SSD and ENT, only one ENT drive is written to, thus, half of the time the write will be faster than if...