22.3. Data Integrity
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## 22.3. Data Integrity

The final problem with disks is that they are unreliable. Although reliability has increased tremendously over the last few years, disk drives are still the most likely core component of a server to fail. When they do, the results can be catastrophic and replacing a failed disk drive and restoring data can result in server downtime.

One approach to this problem is mirroring, or RAID-1, which keeps two copies of the data on different physical hardware. Any write to the volume writes to both disks; a read can be satisfied from either, so if one drive fails, the data is still available on the other drive.

Mirroring has two problems:

- It requires twice as much disk storage as a non-redundant solution.

- Writes must be performed to both drives, so they take up twice the bandwidth of a non-mirrored volume. Reads do not suffer from a performance penalty and can even be faster.

An alternative solution is parity, implemented in RAID levels 2, 3, 4 and 5. Of these, RAID-5 is the most interesting. As implemented in `vinum`, it is a variant on a striped organization which dedicates one block of each stripe to parity one of the other blocks. As implemented by `vinum`, a RAID-5 plex is similar to a striped plex, except that it implements RAID-5 by including a parity block in each stripe. As required by RAID-5, the location of this parity block changes from one stripe to the next. The numbers in the data blocks indicate the relative block numbers.

Figure 22.3. RAID-5 Organization

Compared to mirroring, RAID-5 has the advantage of requiring significantly less storage space. Read access is similar to that of striped organizations, but write access is significantly slower, approximately 25% of the read performance. If one drive fails, the array can continue to operate in degraded mode where a read from one of the remaining accessible drives continues normally, but a read from the failed drive is recalculated from the corresponding block from all the remaining drives.

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