someone19 :
The write time penality is for the system to generate that parity data and store it. So a Raid 5 and a Raid 1 will have the same write performance, possibly slower with raid 5.
This is absolutely not true.
When you write to a RAID-1 array, the RAID controller issues two write operations, one to each drive, in parallel. Both write operations occur at the same time and you have to wait for whichever one takes the longest.
When you write to a RAID-5 array, the RAID controller has to issue two READ operations - one to the data block about to be overwritten and one to the corresponding parity block. The two reads occur at the same time and you have to wait for whichever one takes the longest. THEN the new parity is computed by XORing the old data block, the parity block, and the new data block, and the RAID controller issues two WRITE operations to update both the data and the parity. The two writes occur at the same time and you have to wait for whichever one takes the longest.
For sequential operations smart RAID controllers can alleviate the extra I/Os to some extent by pre-reading and caching data, but still the basic equation is that RAID-5 writes take AT LEAST TWICE as long as RAID-1 writes.
Do NOT use RAID-5 for write-intensive workloads, it's performance under those conditions really sucks.
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Raid 5 isn't a mirror, the last drive in the array is used to store parity information.
Actually, that's RAID-4, which nobody uses these days because the parity drive becomes a bottleneck for all the other drives. In RAID-5 the parity blocks are distributed across all of the drives to balance the I/O load.