Re: which raid level gives maximum overall speed? (raid-10,f2 vs. raid-0)

[Janek Kozicki <janek_listy@wp.pl>]

Keld Jørn Simonsen said:     (by the date of Thu, 31 Jan 2008 02:55:07 +0100)

> Given that you want maximum thruput for both reading and writing, I
> think there is only one way to go, that is raid0.
> 
> All the raid10's will have double time for writing, and raid5 and raid6
> will also have double or triple writing times, given that you can do
> striped writes on the raid0. 
> 
> For random and sequential writing in the normal case (no faulty disks) I would
> guess that all of the raid10's, the raid1 and raid5 are about equally fast, given the
> same amount of hardware.  (raid5, raid6 a little slower given the
> unactive parity chunks).
> 
> For random reading, raid0, raid1, raid10 should be equally fast, with
> raid5 a little slower, due to one of the disks virtually out of
> operation, as it is used for the XOR parity chunks. raid6 should be 
> somewhat slower due to 2 non-operationable disks. raid10,f2 may have a
> slight edge due to virtually only using half the disk giving better
> average seek time, and using the faster outer disk halves.
> 
> For sequential reading, raid0 and raid10,f2 should be equally fast.
> Possibly raid10,o2 comes quite close. My guess is that raid5 then is
> next, achieving striping rates, but with the loss of one parity drive,
> and then raid1 and raid10,n2 with equal performance.
> 
> In degraded mode, I guess for random read/writes the difference is not
> big between any of the raid1, raid5 and raid10 layouts, while sequential
> reads will be especially bad for raid10,f2 approaching the random read
> rate, and others will enjoy the normal speed of the above filesystem
> (ext3, reiserfs, xfs etc).


Wow! Thanks for detailed explanations. 

I was thinking that maybe raid10 on 4 drives could be faster than
raid0. But now it's all logical for me. With 4 drives and raid10,f2
I could get an "extra" reading speed, but not the writing speed. Makes
a lot of sense.

Perhaps it should be added to linux-raid wiki? (and perhaps a
FAQ there - isn't a question about speed a frequent one?)

  http://linux-raid.osdl.org/index.php/Main_Page  


> Theory, theory theory. Show me some real figures.

yes... that would be great if someone could spend some time
benchmarking all possible configurations :-)

thanks for your help!
-- 
Janek Kozicki
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