Accommodating arrays

Flexible Raid Array Products

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Modern storage arrays offer disk types to meet any need--costly Fibre Channel (FC) disks for high-end applications requiring superior performance and availability, and lower-priced SATA disks for less-critical data. The arrays also come with mixed RAID configurations. But selecting the right mix of disks and RAID levels requires understanding the impact of those decisions.

Arrays with mixed RAID support allow users to optimize application cost, performance and availability on a single array. With storage management an ever-escalating cost, the ability to select a single vendor's array to satisfy a multitude of storage scenarios is enticing. In addition to the different types of disk, the new arrays offer simplified management, ease of data migrations from one tier of disk to another, and the ability to increase capacity or improve performance on the fly.

Is RAID 5 all you need?
RAID 5 provides acceptable levels of protection, performance, availability and capacity for most applications. It's becoming the default standard for Fibre Channel disks, but other RAID settings should be considered in certain instances:
Choose RAID 0 for read-and-write I/O-intensive applications that require top-notch performance, but where availability and data protection aren't chief concerns.
Read-intensive applications will perform better under RAID 1.
Video-streaming applications perform better with RAID 3, which handles large block transfers and read/write I/O extremely well.
But don't completely dismiss the other RAID levels. In a few years, 1TB disks are expected to arrive. Putting them in a RAID 5 configuration isn't a problem, but it will take considerable time for a 1TB disk to recover from a failure. This opens up the possibility of another disk in the same RAID group failing, resulting in data loss. A higher degree of protection like RAID 6 may be needed.

RAID 6, which will be available next year, dedicates two disks to parity, rather than one with RAID 5. In other words, two disks can fail in the same RAID group without any data loss. Vendors plan to target this new RAID configuration for environments where data protection and availability are key, but performance isn't critical. RAID 6 should be used in situations where high-capacity, less-reliable disks are used for low-performance applications with data that must remain constantly online and available.

Despite the fact almost every storage array vendor offers at least one array that supports more than one RAID type, vendors don't agree on whether you need more than just one RAID configuration. Nexsan Technologies says RAID 5 is best in almost every data protection situation. EqualLogic Inc. disagrees: It offers RAID 10 or RAID 50, but not RAID 5. Xiotech Corp. wants to see RAID 6 come on the market.

RAID 5, however, continues to gain momentum as the default standard for FC and serial ATA (SATA) disk (see "Is RAID 5 all you need?"). Users find it provides acceptable levels of data protection, disk utilization and performance for most applications. But there are still times when users will want to consider using other RAID levels, especially when using SATA disks. In cases where multiple RAID types are used, administrators need to understand how and where these RAID types get placed internally within the array and the risks associated with mixing and managing multiple RAID types on a single array.

Knowing which RAID implementation to deploy depends on a range of factors:

  • The number of front- and back-end controllers on the array
  • The amount of cache
  • The disk capacity behind each back-end controller
  • The performance requirements of the application using the disk
  • The speed or rpm of each disk behind each controller

Industry benchmarks and vendor documentation will provide statistics and information on array cache, I/O capabilities, the number of front-end FC and iSCSI interfaces, back-end controllers, the type of disk used and internal architecture. Once administrators gather these facts, they can determine the best RAID configuration for their environment.

The right RAID
Storage array vendors allow RAID configurations to be set on each controller or parity group that sits in front of the disks. Using array management software provided by the storage vendor, users can log in and configure any controller on the array with any of the RAID settings the controller supports. Users may also change an application's underlying RAID configurations on the fly, assuming they have the vendor's licensed software and a spare disk group. Arrays such as EMC Corp.'s Clariion, IBM Corp.'s DS4000 and Hitachi Data Systems' (HDS) TagmaStore offer software that allows users to move the data from a disk group configured as RAID 1 to a disk group configured as RAID 5 without application downtime.

Yet selecting a specific RAID type is becoming less of an issue on high-end monolithic and modular arrays as users increasingly choose RAID 5. HDS reports that more than 85% of its arrays now get configured as RAID 5 because users find that RAID 5 provides acceptable tradeoffs between availability, capacity, data protection and performance when compared to other RAID configurations. However, not every storage array vendor implements RAID 5 the same way. Here are some examples of how they differ:

  • Modular models sold by BlueArc Corp., Hewlett-Packard Co., IBM and Silicon Graphics Inc. (SGI) use RAID controllers supplied by Engenio Information Technologies, which offers two different types of disk controllers, the 5884 and 28XX models. The 5884 controller is ASIC-based and used primarily with FC disks. Engenio bases its lower-end 28XX models on Intel Corp.'s XScale chip, and it's used primarily in its SATA arrays.
  • The newest RAID controllers soon to show up on IBM's DS4000 will support Emulex Corp.'s switch-on-a-chip technology. This approach provides a dedicated path between the controller and each disk drive, as opposed to a shared path between the controller and all of the disk drives behind it.
  • An increasing number of arrays now support global hot spares. These are disk drives not tied to any disk group that can replace a disk in a RAID 5 configuration that fails.
  • HDS is among a growing number of vendors implementing RAID5+, where parity is striped across all of the volumes in the RAID group. This helps to eliminate most of the write penalty associated with RAID 5.

This was first published in November 2004

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