Why is it not recommended to use RAID 5 for a log file?
Transaction log writes, when they occur, are synchronous operations, that is, the activity that has caused a log write must wait until log I/O completes before continuing to do whatever it is doing. As a result log writes are very sensitive to the write throughput of the underlying storage.

As you have mentioned, every write to a RAID-5 device has an overhead1 of calculating and writing a parity block in addition to the data block(s). However small, this extra work RAID-5 performs on each write operation is the reason behind the recommendation to not use RAID-5 for log storage.



1 - More details in this Q&amp;A