HP DL360 Drive technology overview - Page 9

are connected to individual SAS-2 channels. In these cases, the performance limiter is the drive throughput, not the speed of the SAS links. The performance benefits of SAS-2 and its faster link speed becomes important when constructing SAS fabrics and larger drive arrays using SAS expanders that support the SAS-2 standard. With these configurations, the throughput of multiple drives is often aggregated across a single SAS-2 channel, thus taking full advantage of the additional SAS-2 bandwidth for increased overall performance. The improved bandwidths of SATA 3.0 and SAS-2 are also important when considering the emerging category of solid state drives (SSDs). Second generation server SSDs using the SATA interface are already capable of delivering 230 MB/s of sustained throughput, almost equal to the bandwidth of a SATA 3.0 link. Third generation SAS SSDs scheduled for 2010 are expected to support 500 - 600 MB/s throughput and thus will be capable of consuming the entire bandwidth of a single SAS-2 link. Improved performance and reliability with RAID Drives do fail; therefore, storing data on a single disk drive creates a risk of data loss. HP recommends always using some form of fault-tolerant RAID across multiple drives. RAID strategies can be characterized by how they achieve data reliability (how parity or other error correction data is distributed across the array), the minimum number of drives required, and data storage efficiency. The performance of multiple drives is better than the performance of a single drive. The choice of RAID strategy and how it is implemented affect read performance, write performance, and robustness in the face of hardware failures:  RAID 0 - striping to two or more disks; no redundancy, performance improvement only  RAID 1 - mirroring; duplicates same data on two disks; redundancy and potential performance improvements  RAID 1 + 0 - mirroring and striping; redundancy and performance improvement  RAID 5 - block striping with distributed parity; three or more drives; fault tolerance  RAID 6 - block level striping with dual distributed parity; three or more drives, increased fault tolerance Both read performance and write performance also vary with the workload; that is, whether I/O (many small data units) or bandwidth (fewer, large data units) predominates. Advanced controllers Advanced controllers, such as the HP Smart Array, decouple the logical disks seen by applications from the physical devices used to implement the disk subsystem. These controllers include both hardware and software. A single logical disk (as seen by an application) may be mapped onto an array of multiple physical disks. This approach provides greatly enhanced flexibility, expandability, maintainability, and performance. Smart Array controllers are available for SAS, SATA, and SCSI interfaces. For further details, see the technology brief entitled "HP Smart Array Controller Technology," at http://h20000.www2.hp.com/bc/docs/support/SupportManual/c00687518/c00687518.pdf. Solid state drives for servers Solid state drives represent a new type of storage technology that is now mature enough to be used for server storage requirements. Unlike traditional disk drives, SSDs store data to, and retrieve data from, NAND flash memory. Until recently, storage devices based on flash memory could not meet the performance or reliability requirements for use in server environments. By taking advantage of 9