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RAID-5 Write Penalty

"...If you later modify the data block it recalculates the parity by subtracting the
old block and adding in the new version then in two separate operations it
writes the data block followed by the new parity block. To do this it must
first read the parity block from whichever drive contains the parity for
that stripe block and reread the unmodified data for the updated block from
the original drive. This read-read-write-write is known as the RAID5 write
penalty since these two writes are sequential and synchronous the write
system call cannot return until the reread and both writes complete, for
safety, so writing to RAID5 is up to 50% slower than RAID0 for an array of
the same capacity. (Some software RAID5's avoid the re-read by keeping an
unmodified copy of the orginal block in memory.)"

RAID-5 Drive Failure
"Now if a drive in the RAID5 array dies, is removed, or is shut off data is
returned by reading the blocks from the remaining drives and calculating
the missing data using the parity, assuming the defunct drive is not the
parity block drive for that RAID block. Note that it takes 4 physical
reads to replace the missing disk block (for a 5 drive array) for four out
of every five disk blocks leading to a 64% performance degradation until
the problem is discovered and a new drive can be mapped in to begin
recovery."

Raid-5 Failure Rate Increases
As the number of disks in a RAID 5 group increases, the Mean Time Between Failures (MTBF, the reciprocal of the failure rate) can become lower than that of a single disk.

Why RAID 5 stops working in 2009

HTH,

HAND.