Creating RAID 1 (mirror) volumes may improve read performance; write performance is always degraded.
Creating RAID 1 (mirror) volumes 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.
Creating RAID 1 (mirror) volumes 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, RAID 1 volumes degrade overall performance. However, the write degradation with RAID 1 volumes is substantially less than the typical RAID 5 write penalty (which can be as much as 70 percent).
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. However, 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.