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Automation Server Performance Enhancement
How do you go about fixing lackluster server performance and/or getting the most from what you have? Poor disk I/O performance can result from outdated or incorrect disk controller drivers, less than optimum setup of the RAID array, or a defective controller. For CBAS servers, we tend to use a RAID 1 (mirror) or a RAID 1 with duplexing (see Fig. 2) for the operating system. RAID 1 arrays consist of two drives in a mirror arrangement and a capacity of (n/2) X. For audio storage we most often use a RAID 5 array, which takes us to the intersection of performance, reliability and efficiency. A RAID 5 array consists of several hard drives (at least three) with a capacity of (n-1) X, where n equals the number of hard drives and X is the individual hard drive capacity. A RAID 5 usually has one or more drives declared to be a global hot spare or dedicated hot spare for failover.
Note that efficiency of surface real estate improves in a RAID 5 array as the number of drives in the array increases. To illustrate this point, consider a RAID 5 consisting of the minimum three drives. Let's suppose that our drives have a capacity of 73GB. Using the formula above, our RAID array would have a capacity of 73GB(3-1), which equals 146GB. Our efficiency of surface real estate is 67 percent. If we have an array of 14 73GB drives, two of which are used for global hot spares (leaving 12 for data), we have a capacity of 73GB (12-1), which equals 803GB. This implies an efficiency of surface real estate for this arrangement of 803GB/876GB × 100 percent, which equals approximately 92 percent. This is one of the benefits of RAID 5 versus other RAID arrangements.
We want to make certain that our RAID arrays are up to standards in order to realize best performance. For a SCSI (Small Computer System Interface) system, you would want drives that spin at least at 7,200rpm/Ultra 320 interface, an Ultra 320 SCSI controller card with 128MB or more cache on-board. You really don't want to skimp here. The SCSI controller card may either be of the non-RAID or RAID type. If the card does not support RAID, then the RAID array will be set up under the operating system. A RAID type controller card will generally support a variety of RAID levels such as: RAID 0, 1, 5 and 10. Software RAID (set up under the operating system) and hardware RAID have advantages and disadvantages.
In most cases hardware RAID is the optimum choice. In this case, it is important to properly set up the array for best performance using the RAID controller's bios settings. Key settings are: RAID stripe size, write-back and read-ahead. RAID stripe size refers to the width of the data stripe in the array, and is not connected to the block size or the size of the allocation units formatted under NTFS (N.T. File System used with Windows).
I mention this because this does cause some confusion, as some administrators believe they should set the array stripe size to be the same as the block size. The optimum block size to set before formatting a drive with NTFS is often a setting recommended by your automation system vendor. For use with streaming music, the maximum setting with NTFS of 64KB is often used (but this may vary with the vendor). In any event, the array stripe size can affect performance of the array quite significantly. Some experts say the bigger the stripe size, the better. I have found 128KB stripe size to be the best for performance with heavily used file servers with multiple clients (as in CBAS operation). That said, go with your vendor's suggestions.
Write back will improve the RAID array's performance by writing data to cache first, then to the disks. This allows the CPU(s) to command a write of the data and then move on, allowing the RAID controller to process the write as hardware timing permits. It's very important to maintain the RAID system's battery when using this option in order to avoid data loss. With regard to the read-ahead setting, you'll find you have three choices: Read-ahead, adaptive read-ahead and no read-ahead.
Read-ahead works like this: A block is read from the disk, and then additional sequential blocks are read in the hope that these will most likely be required next. Adaptive read-ahead uses an algorithm that will use read-ahead until such time as the last two or so read operations were not sequential. For file servers used for audio streaming it's generally best to turn off read-ahead since file block reads are likely not to be sequential. This setting may be shown as normal on your RAID controller.
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