mdadm on huge raid sets

mdadm on huge raid sets

[Erich Weiler]

Greetings all-

Quick question.  We've got one of these new Sun x4500's, it is basically 
a machine with 48 disks in it, no hardware RAID controller, so you have 
to use software RAID, in our case mdadm because we're running RHEL 4.4.

When we try to create the raid set (RAID6), we're getting an error: 
"Invalid number of devices" (47 devices).  We tried lowering the number, 
and eventually it worked when we brought the number down to 27.  But 
looking at the source code, it looks like it supports up to 256 devices 
for RAID6... ?  Any insight would be much appreciated!!

Thanks a million,
-erich

Re: mdadm on huge raid sets

[Neil Brown]

On Tuesday May 22, weiler@soe.ucsc.edu wrote:
> Greetings all-
> 
> Quick question.  We've got one of these new Sun x4500's, it is basically 
> a machine with 48 disks in it, no hardware RAID controller, so you have 
> to use software RAID, in our case mdadm because we're running RHEL 4.4.
> 
> When we try to create the raid set (RAID6), we're getting an error: 
> "Invalid number of devices" (47 devices).  We tried lowering the number, 
> and eventually it worked when we brought the number down to 27.  But 
> looking at the source code, it looks like it supports up to 256 devices 
> for RAID6... ?  Any insight would be much appreciated!!

Use "--metadata=1" to mdadm.  The 0.90 superblock has a limit of 27
devices. version 1 has no fixed limit, but the code might not support
more than 384 very well.

> 
> Thanks a million,

That would be a megathank :-?

NeilBrown