User manual DOT HILL BEYOND SOFTWARE RAID BROCHURE

[. . . ] RAID has been used to improve system read/write performance as well. An acronym for Redundant Array of Independent Disks (originally Redundant Array of Inexpensive Disks) RAID is a computer data storage scheme that can divide and replicate data among multiple hard disk drives and can be used to increase data reliability or increase input/output performance. While this is not a tutorial on RAID technology, the most common forms of RAID are: RAID 0 (Data Striping) RAID 1 (Data Mirroring) RAID 5 (Block Striping w/ Striped Parity) RAID 6 (Block Striping w/ Multiple Striped Parity) RAID 10 (or RAID 1+0 ­ Mirrored Striped Array) RAID 50 (or RAID 5+0 ­ RAID 5 Striped Array) Traditionally, RAID has been implemented in the controller of an external disk array or up on an application server via an add-in RAID adapter card. With faster processors and the multi-core processors of today, RAID now runs efficiently on an application server in concert with the operating system; this is usually referred to as "software RAID. " Regardless where it runs, it is important to understand that in essence all RAID is done in software. [. . . ] This maximizes the investment of the motherboard and makes good use of idle resources o Uses CPU cycles, internal bus, main memory and onboard I/O ports o A CPU and memory upgrade can be less expensive and more effective than a hardware controller Scaled performance ­ Overall RAID speed increases as host CPU count (multi-thread, multicore) increases, following current market trends. Server slot management ­ PCI slots can be used for other function (e. g. , SAS tape adapter, FC HBA) Disadvantages Key disadvantages of software RAID include: Consumes and places additional load on host resources (i. e. , CPU, memory and internal bus) No boot protection ­ While some pure software RAID solutions have boot protection, many do not. For those that don't, disk failure prior to loading the operating system during the boot process can result in an inoperable system. RAID1/5 Reliability ­ For RAID1 and RAID5 (see RAID5 write hole) running on a host with no NVRAM, system power failures or hardware or software errors occurring for in-progress write operations can result in mirror or parity consistency issues, resulting in corrupt data. Write performance ­ Software RAID is always in write-through mode which extends the writecommit back to the host (vs. Limited expansion capability o 10s not 100s of disk drives o The more drives the more burden on host resources 5 Hardware RAID Adapters Hardware RAID adapters have their own processor and memory to run the RAID application. In essence it is an entire computer unto itself dedicated to the RAID application, and acts as a conduit between the server and the array off-loading this task from the host system. They are separate RAID adapter cards that are installed in available server expansion slots. A RAID adapter manages the system disk drives and then presents the logical drives to the server. The server sees the array or arrays as just one or more very fast hard disks; the RAID is completely hidden from the host machine. Hardware RAID adapters usually offer a full range of RAID options (i. e. , 0, 1, 5, 6, 10, 50). Advantages The salient advantages of hardware RAID adapters include: Improved host system and I/O performance ­ The demand on host system resources (CPU, memory and internal bus) are mitigated by offloading the RAID function to a hardware RAID adapter. This is especially important when one or more CPU intensive applications are running, or as in a virtualized server environment where the aggregation of multiple physical servers into multiple virtual servers running on one physical server places significant demands on the internal CPU, memory and internal bus resources. Boot protection ­ The boot image can be RAID protected prior to loading the operating system mitigating corruption should a disk failure occur during the boot process. RAID1/5 Reliability ­ Battery-backed write cache ensures mirror and parity consistency in the event of system failure for in-progress writes. Improved write performance ­ With a cache memory, write-back caching can be enabled speeding the write-commit back to the host; as opposed to write-through mode which extends the write-commit back to the host. Scalability ­ The number of drives supported is typically much greater than with software RAID. Advanced Functions ­ Hardware RAID adapters typically serve as a platform for advanced functions such as online RAID level migration (RLM), on-line capacity expansion (OCE) and SAS expansion. Disadvantages Key disadvantages of hardware RAID adapters include: Higher system cost o Acquisition Costs ­ Extra cost of the hardware component o Operating Costs ­ Extra power and cooling costs of the hardware component No controller spanning ­ Hardware RAID adapters can only manage disks dedicated to that controller. This can result in multiple RAID stacks (one for core logic SATA I/O ports and one for the SAS IOC ports) and multiple storage pools managed independently. Fixed performance ­ I/O performance may not scale as host processing power increases Fixed feature set o May have to purchase functionality you don't want o Will have to buy a new HBA for upgraded or new functionality 6 Virtual RAID Adapters With advances in processors, memory and bus technologies a new form of RAID is emerging which bridges the gap between pure software RAID and hardware RAID adapters. Virtual RAID adapters (VRAs) combine the key advantages of pure software RAID with many of the key advantages of hardware RAID adapters. [. . . ] With its patented technology and award-winning SANnet II® and RIO XtremeTM families of storage and its Dot Hill Storage Services, Dot Hill makes storage easy. Headquartered in Carlsbad, Calif. , Dot Hill has offices in China, Germany, Israel, Japan, Netherlands, United Kingdom and the United States. More information is available at http://www. dothill. com. Worldwide Headquarters: Dot Hill Systems Corp. 100 Carlsbad, Ca 92008 © 2009, Dot Hill Systems Corp. , All rights reserved. [. . . ]