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Spotlight on midrange arrays

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Use caution when mixing Fibre Channel and SATA Drives

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Randy Kerns, a senior partner at Evaluator Group, Greenwood Village, CO, recommends users consider the following when mixing Fibre Channel (FC) and SATA drives in a single midrange array:
Managing array resources. Most midrange implementations don't allow administrators to control the allocation of controller resources for the disk drive types. As a result, cache, processor resources and internal ports are all managed as if the disk types were the same.
Controlling data placement. If controlling whether data gets placed on FC or SATA drives is a manual process, the administrative costs and risks associated with managing this process may be higher than just putting everything on more expensive FC disk drives.
Disk drive duty cycles. The reliability of FC and SATA disk drives is measured in very different ways. The failure rates of high-performance FC disk drives are generally benchmarked based upon 24-hour-a-day usage, while availability measurements for SATA drives are generally based on two to four hours of usage a day.
Disk drive processors. FC disk drives contain two processors--one to handle the command processing for data access and to set up data transfer, and the other to control the servo mechanism to keep the head aligned with the data to ensure correct and optimal data transfer. SATA drives contain only one processor to handle the servo positioning, command processing and data transfer. Under heavy loads or multiple requests to the same disk for data, performance will be significantly impacted on SATA disks with only one processor to handle the multiple I/O requests.
Midrange arrays arrive
Midrange arrays are changing the way companies manage their data and business. Gone are the days when pricey monolithic arrays ruled the data center. Cost is key. Companies no longer need to spend millions on a monolithic array for storage applications that a midrange array can handle for substantially less money.

As this Special Report reveals, midrange arrays offer a wide choice of features that can fulfill almost every storage need. Because the choices are so varied in the midrange array category, some homework is required to find the right array at the right price for specific storage requirements. Applications will dictate your requirements, but a midrange array can offer the best fit whether an application requires low-cost, high-capacity disk, the ability to mix and match different kinds of disk and RAID within an array, replication between monolithic and midrange arrays, or the ability to virtualize other storage arrays.

For starters, the type of midrange array hardware and software features that best serve a company's storage requirements must be determined. These features include:

  • Type of disk drives
  • Type and number of RAID controllers
  • Amount of cache
  • Number and type of front-end ports
  • Software such as array management, volume management, snapshot and mirroring

Disk drive support
A compelling force behind the deployment of midrange arrays is their ability to support disk drives of almost any type, size or number. The lowest-priced configurations are approximately $5,000 per terabyte and show up in midrange arrays, such as EqualLogic Inc.'s PS200E and Isilon Systems Inc.'s IQ 2250, that support only SATA drives. Products like 3PAR Inc.'s InServ S400, IBM's TotalStorage DS6800 and Sun Microsystems Inc.'s StorEdge 6920 support only high-performance Fibre Channel (FC) drives, and are priced at around $50,000 per terabyte. Still other companies, like Hitachi Data Systems (HDS) Corp., offer arrays that support either SATA or FC drives.

The trend, however, is for midrange arrays to support both high-performance FC drives and high-capacity SATA drives behind the same front-end host interface. Vendors such as EMC Corp. and HDS--that initially supported only FC disk drives in their Clariion and Thunder series arrays, respectively--now include SATA support within these arrays and allow users to mix FC and SATA disk drives on the same system. 3PAR, IBM and Sun, who don't currently support SATA drives within their midrange systems, plan to add that support this year.

The ability to mix high-performance FC and high-capacity SATA drives within a midrange array gives administrators the flexibility to put the right data on the right kind of disk. For example, FC disk drives are designated for a high-performance database or file system apps, while the high-capacity, lower-cost SATA disk drives are used for apps calling for disk-to-disk backup, snapshots, virtual tape libraries, e-mail archives and fixed content.

Because different capacities and speeds exist for high-performance FC disk drives, users need to weigh several factors to come up with their best choice. IBM says that as a general rule, the smaller and faster the disk drive, the better the performance. HDS finds that 15,000 rpm FC disks will provide up to a 15% increase in performance over 10,000 rpm FC disks in random read environments.

One way to improve performance on slower disks is to distribute the data volumes across multiple disks. Chris Berthaut, the open-systems storage team manager with Hibernia National Bank in New Orleans, says his team uses this feature on the nine Xiotech Corp. Magnitude Classics they manage. "The virtualization [feature] was a big factor in the decision to buy Xiotech arrays since it allowed us to easily stripe data across disks on their arrays," says Berthaut.

A final area that may be overlooked is how the array recovers from failed disk drives and how easy it is to replace the faulty drives. Many midrange arrays, including HDS' Thunder 9520V, have a "call home" feature that reports a disk drive failure or a drive that's on the verge of failing. HDS reports that approximately 95% of the disk drives replaced on HDS' Thunder arrays "soft fail"--the disk exceeds an error threshold and is swapped out before the disk physically fails. Thunder copies the data from the poorly performing disk to a spare in the system. This approach minimizes performance degradation and speeds up recovery time since a copy operation runs faster than a RAID 5 rebuild.

This was first published in March 2005

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