DB2 Version 9.7 for Linux, UNIX, and Windows
DB2 Version 9.7 for Linux, UNIX, and Windows
If you are concerned about the possibility of damaged data or logs due to a disk crash, consider the use of some form of disk fault tolerance. Generally, this is accomplished through the use of a disk array, which is a set of disks.
A disk array is sometimes referred to simply as a RAID (Redundant Array of Independent Disks). Disk arrays can also be provided through software at the operating system or application level. The point of distinction between hardware and software disk arrays is how CPU processing of input/output (I/O) requests is handled. For hardware disk arrays, I/O activity is managed by disk controllers; for software disk arrays, this is done by the operating system or an application.
In a hardware disk array, multiple disks are used and managed by a disk controller, complete with its own CPU. All of the logic required to manage the disks forming this array is contained on the disk controller; therefore, this implementation is operating system-independent.
There are several types of RAID architecture, differing in function and performance, but only RAID level 1 and level 5 are commonly used today.
RAID level 1 is also known as disk mirroring or duplexing. Disk mirroring copies data (a complete file) from one disk to a second disk, using a single disk controller. Disk duplexing is similar to disk mirroring, except that disks are attached to a second disk controller (like two SCSI adapters). Data protection is good: Either disk can fail, and data is still accessible from the other disk. With disk duplexing, a disk controller can also fail without compromising data protection. Performance is good, but this implementation requires twice the usual number of disks.
RAID level 5 involves data and parity striping by sectors, across all disks. Parity is interleaved with data, rather than being stored on a dedicated drive. Data protection is good: If any disk fails, the data can still be accessed by using information from the other disks, along with the striped parity information. Read performance is good, but write performance is not. A RAID level 5 configuration requires a minimum of three identical disks. The amount of disk space required for overhead varies with the number of disks in the array. In the case of a RAID level 5 configuration with 5 disks, the space overhead is 20 percent.
RAID level 1+0 (10) involves mirroring and striping the data across at least two disks. Mirroring writes the data to two or more disks at the same time which gives you the same fault tolerance as RAID level 1. Striping breaks the data into blocks and each block is written down to a separate disk drive. This achieves high I/O performance by spreading the I/O load across many channels and drives but RAID level 1+0 reduces the effective disk space in half as it mirrors all the data. RAID Level 10 requires a minimum of 4 drives to implement.
RAID level 0+1 is implemented as a mirrored array whose segments are RAID 0 arrays and has the same fault tolerance as RAID level 5. This gives high I/O rates by spreading the I/O load across many channels and drives. RAID level 0+1, however, is not to be confused with RAID level 1+0. A single drive failure will cause the whole array to become, in essence, a RAID level 0 array.
When using a RAID (but not a RAID level 0) disk array, a failed disk will not prevent you from accessing data on the array. When hot-pluggable or hot-swappable disks are used in the array, a replacement disk can be swapped with the failed disk while the array is in use. With RAID level 5, if two disks fail at the same time, all data is lost (but the probability of simultaneous disk failures is very small).
You might consider using a RAID level 1 hardware disk array or a software disk array for your logs, because this provides recoverability to the point of failure, and offers good write performance, which is important for logs. To do this, use the mirrorlogpath configuration parameter to specify a mirror log path on a RAID level 1 file system. In cases where reliability is critical (because time cannot be lost recovering data following a disk failure), and write performance is not so critical, consider using a RAID level 5 hardware disk array. Alternatively, if write performance is critical, and the cost of additional disk space is not significant, consider a RAID level 1 hardware disk array for your data, as well as for your logs.
For detailed information about the available RAID levels, visit the following web site: http://www.acnc.com/04_01_00.html
A software disk array accomplishes much the same as does a hardware disk array, but disk traffic is managed by either the operating system, or by an application program running on the server. Like other programs, the software array must compete for CPU and system resources. This is not a good option for a CPU-constrained system, and it should be remembered that overall disk array performance is dependent on the server's CPU load and capacity.
A typical software disk array provides disk mirroring. Although redundant disks are required, a software disk array is comparatively inexpensive to implement, because costly disk controllers are not required.