Re: striping of a 4 drive raid10

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Sun, Jan 27, 2008 at 08:11:35PM +0000, Peter Grandi wrote:
> >>> On Sun, 27 Jan 2008 20:33:45 +0100, Keld Jørn Simonsen
> >>> <keld@dkuug.dk> said:
> 
> keld> Hi I have tried to make a striping raid out of my new 4 x
> keld> 1 TB SATA-2 disks. I tried raid10,f2 in several ways:
> 
> keld> 1: md0 = raid10,f2 of sda1+sdb1, md1= raid10,f2 of sdc1+sdd1, md2 = raid0
> keld>    of md0+md1
> keld> 2: md0 = raid0 of sda1+sdb1, md1= raid0 of sdc1+sdd1, md2 = raid01,f2
> keld>    of md0+md1
> keld> 3: md0 = raid10,f2 of sda1+sdb1, md1= raid10,f2 of sdc1+sdd1, chunksize of 
> keld>    md0 =md1 =128 KB,  md2 = raid0 of md0+md1 chunksize = 256 KB
> keld> 4: md0 = raid0 of sda1+sdb1, md1= raid0 of sdc1+sdd1, chunksize
> keld>    of md0 = md1 = 128 KB, md2 = raid01,f2 of md0+md1 chunksize = 256 KB
> 
> These stacked RAID levels don't make a lot of sense.
> 
> keld> 5: md0= raid10,f4 of sda1+sdb1+sdc1+sdd1
> 
> This also does not make a lot of sense. Why have four mirrors
> instead of two?

My error, I did mean f2.

Anyway 4 mirrors would make the disk 2 times faster than 2 disks, and given disk
prices these days this could make a lot of sense.

> Instead, try 'md0 = raid10,f2' for example. The first mirror of
> will be striped across the outer half of all four drives, and
> the second mirrors will be rotated in the inner half of each
> drive.
> 
> Which of course means that reads will be quite quick, but writes
> and degraded operation will be slower.
> 
> Consider this post for more details:
> 
>   http://www.spinics.net/lists/raid/msg18130.html

Thanks for the reference.

There is also more in the original article on possible layouts of what
is now known as raid10,f2

http://marc.info/?l=linux-raid&m=107427614604701&w=2

including performance enhancements due to use of the faster outer
sectors, and smaller average seek times because you can seek on only
half the disk.

best regards
keld
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