Tuning RAC on RAID5

RAID 5 and RAC

Many storage administrators tend to favor RAID 5 that stripes data across multiple disk units (more on striping in a little bit) and also distributes parity information among those disk units. The computed parity is used to reconstruct data in the event one disk unit is unavailable. A typical RAID 5 configuration would use approximately fifteen to twenty percent of the disk space in the RAID group to support parity bits, depending on the number of disk units in that group.

While using less space to support redundancy compared to RAID1, The downside to RAID 5 is the time for write operations to complete. When a block of data is changed on RAID 5 disk, the disk needs to read the current value and read the current parity information. Then the new value is written to disk and then the new parity is written to disk. Two write operations occur for every write request received. As such, write operations on RAID 5 can take twice as long to complete compared to RAID 1.

It is common to see 1 terabyte, or larger, high-capacity disk drives being sold today. With the larger capacity drives used in the disk subsystem, RAID 6 is becoming more popular. Where RAID 5 protects against loss of one disk in the RAID group, RAID 6 protects against the loss of two disks in the RAID group by using double parity.  Using double parity adds one more write operation than seen in RAID 5 so RAID 6 should not be used to support Oracle RAC databases.

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