RAID5 gets a bad rap

[Bill Davidsen]

Gordon Messmer wrote:
> Chris Tyler wrote:
>> On Tue, 2008-12-30 at 01:02 -0800, Gordon Messmer wrote:
>>> That's not quite it.  RAID 5 performance suffers because every write 
>>> requires that the entire block that's being written be read from 
>>> every drive in the array, parity calculated, and then the data and 
>>> parity written out.  For each block written, the array has to do N 
>>> reads plus two writes.
>>
>> You don't have to read all of the drives -- just the block you're
>> updating and the parity block. XOR the old data you're about to
>> overwrite with the parity block and the new data and you'll have the new
>> parity block. Total activity: two reads plus two writes.
> 
> I've understood that to be the case, but while watching the drive 
> activity lights on RAID5 arrays, it seems like I always see the entire 
> set flash at the same time.  I guess I'll have to investigate that 
> further to find out why.  Thanks.
> 
>>> RAID 5 tends to be most appropriate when you're trying to get as much 
>>> disk space as you can with the lowest cost, you won't be running 
>>> multiple simultaneous jobs on the same disk array, and when you'll be 
>>> collecting data at a rate that's relatively low.
>>
>> I'd say the other way around -- RAID 5 is poor at small writes (hence
>> the OP's comments about database updates), but very nearly approaches
>> RAID-0 speeds when reading or writing large quantities of sequential
>> data.
> 
> Your assertion ignores the fact that filesystems themselves are, in 
> fact, databases.  Real-world experience with many production systems and 
> many workloads has convinced me to use RAID 5 as rarely as possible. 
> Even when I'm forced to use it, I generally choose a RAID 5+0 
> configuration as I get much better performance.
> 
Or you might want to read the man pages for md and mdadm. RAID10 is faster 
(assuming you use the "far 2" config). No, RAID10 is not another name for RAID1+0...

-- 
Bill Davidsen <davidsen at tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot