Donald K. Burleson Oracle RAC Tips

Mirrored Stripes or Striped Mirrors

This RAID level is often misunderstood. Which should you use, RAID 10 or RAID 0+1? Let us examine Fig 5.5 and see exactly how they work.

Fig 5.5 Mirrored Stripes and Striped Mirrors

In the case of RAID 0+1 (also called mirrored stripe), striping is implemented in the lowest position, meaning from a group of drives. A stripe is formed and then two such stripes are mirrored resulting in a logical drive. With this method, the loss of any drive makes the entire stripe invalid and the stripe is detached from the mirror pair. When the failed disk is replaced, the entire stripe must be brought up to date. Recovering the entire stripe can take a substantial amount of time as all the stripe members need to update the data.

With RAID 1+0 (or 10), mirroring is implemented at the lowest position. A group of mirrored drives are used to create a stripe. RAID 10 (also called a striped mirror) is the superior method. If a disk fails in a striped mirror layout, only the failing disk is detached, and only that portion of the volume loses redundancy. When the disk is replaced, only a portion of the volume needs to be recovered. As shown in Fig 5.5, mirrored drives are independent units. When a disk is lost, it affects only one mirrored pair.

Thus, compared to a mirrored-stripe, a striped-mirror offers more tolerance to disk failure. If a disk failure occurs, the recovery time is shorter for a striped-mirror layout.

A short summary of the RAID levels is presented in Table 5.1.

Table: 5.1 Strengths and Weaknesses of Various RAID Levels

One thing to remember when calculating the number of disks required for a particular RAID configuration is not just the storage capacity required, but to also take into account the needed I/O requirements. On some configurations the added parity writes and other overhead associated with RAID5 for example, can reduce the I/O capacity of the drive set by up to 50% (disks are limited to around 110-120 I/Os per second maximum I/O capacity for linear read/write activity. For random read/write activity this can drop to 90 I/Os per second or less.) When you take I/O capacity into account, RAID10 or 01 becomes less costly with the modern RAID controllers since it essentially doubles I/O capacity even if it halves storage capacity.