Mirroring improves read performance only in threaded or asynchronous I/O situations; if there is just a single thread reading from the metadevice, performance will not improve.
Mirroring degrades write performance by about 15-50 percent, because two copies of the data must be written to disk to complete a single logical write. If an application is write intensive, mirroring will degrade overall performance. However, the write degradation with mirroring is substantially less than the typical RAID5 write penalty (which can be as much as 70 percent). Refer to Figure 7-1.
Note that the UNIX operating system implements a file system cache. Since read requests frequently can be satisfied from this cache, the read/write ratio for physical I/O through the file system can be significantly biased toward writing.
For example, an application I/O mix might be 80 percent reads and 20 percent writes. But, if many read requests can be satisfied from the file system cache, the physical I/O mix might be quite different--perhaps only 60 percent reads and 40 percent writes. In fact, if there is a large amount of memory to be used as a buffer cache, the physical I/O mix can even go the other direction: 80 percent reads and 20 percent writes might turn out to be 40 percent reads and 60 percent writes.