Re: RAID 5 of RAID 5's?

[Andy Smith <andy@lug.org.uk>]

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On Mon, Jun 13, 2005 at 09:15:39AM +0200, Laurent CARON wrote:
> Dan Stromberg a ?crit :
> >>>Has anyone constructed a RAID 5 of RAID 5's using mdadm on a linux
> >>>system?
> >>>
> >>>Was it reliable?
> >>>
> >>>How large was it?
> >
> RAID 10 is IMHO a bit more efficient.
> 
> Raid 5 means at least 9 disks:
> Usable capacity: 4 Disks
> Read speed: Good
> Write speed: poor

With a low number of disks, you would indeed get the same or less
capacity with RAID 5 + RAID 5 as compared to RAID 10, but as the
number of disks gets higher, doesn't the capacity of RAID 5 on top
of RAID 5 (we need a name for this, is RAID 55 technically correct?)
get better?

For example, 30 disks, organised as 10 3-disk RAID 5s, have 9*2 or
18 disks of capacity, whereas a RAID 10 of same would be 15 disks.

The write performance seems bad though, and maybe the rebuild time
as well.

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