Archive for the ‘Data Storage’ Category

Tuning Data Access

Performance may suffer if there are wait events caused by file accesses. In general, one should avoid placing files on distributed parity RAID systems such as RAID5. The overhead generated during writes to such file systems generally presents a performance bottleneck as the system use increases, particularly for sequentially written files such as redo log files. Favor the use of RAID 0+1 to support both the mirroring and striping of data without introducing performance bottlenecks.


Disk Mirroring:

In data storage, disk mirroring or RAID1 is the replication of logical disk volumes onto separate physical hard disks in real time to ensure continuous availability. A mirrored volume is a complete logical representation of separate volume copies.

In addition to providing an additional copy of the data for the purpose of redundancy in case of hardware failure, disk mirroring can allow each disk to be accessed separately for reading purposes. Under certain circumstances, this can significantly improve performance as the system can choose for each read which disk can seek most quickly to the required data. This is especially significant where there are several tasks competing for data on the same disk, and thrashing (where the switching between tasks takes up more time than the task itself) can be reduced. This is an important consideration in hardware configurations that frequently access the data on the disk.

Data Striping

In computer data storage, data striping is the segmentation of logically sequential data, such as a single file, so that segments can be assigned to multiple physical devices (usually disk drives in the case of RAID storage, or network interfaces in the case of Grid-oriented Storage) in a round-robin fashion and thus written concurrently.

This technique is useful if the processor is capable of reading or writing data faster than a single disk can supply or accept it. While data is being transferred from the first disk, the second disk can locate the next segment. Striping can be either of type coarse or fine.