[nmglug] raid 10 vs 5

[Nick Frost]

On Sep 9, 2009, at 10:50 AM, Gary Sandine wrote:
>
> I would probably make a RAID 6 array because I think that should
> always survive a double disk failure.  RAID 5 with a hot spare could
> fail if a second disk fails while the hot spare is being included
> (the rebuild can take a while for a 2 TB array), and RAID 10 could
> fail if one of the mirrors loses its second disk before a RAID 1
> component finishes mirroring onto a new disk.
>
> I have only done RAID 6 with hardware RAID cards though (by Areca
> and 3ware).  I think the Linux kernel software RAID driver supports
> RAID 6 now.

We (a contemporary of mine) and I have observed that Linux software  
RAID works well but in cases of heavy I/O the RAID replication can  
fall behind and overwhelm the machine under some circumstances.  So,  
it depends on the application.   I've used software RAID with good  
results, but on systems with light disk I/O.

We have eight HP DL380 G5 servers with MSA 70 arrays at one of my  
jobs, they do RAID-6 on the heads and the MSA arrays.  A total of 232  
10,000 RPM 2.5" SAS drives in all I think.  I really like the hardware  
RAID-6 because a couple times a year a few drives need replacement and  
I just order them from HP, pop them in, and the array rebuilds.   
Pretty much plug and play.

An advantage of software RAID, RAID-1 for example, is the ability to  
specify the filesystem type and mount a filesystem pulled from a  
RAID-1 array, whereas with many hardware RAID solutions reading the  
drives directly may not be possible.

It's been a few years but we used 3Ware SATA RAID cards at a previous  
job with very good (performance) results.  The 3Ware cards allow for a  
hot spare that will automatically be swapped in if a drive is marked  
as failed or degraded.

-Nick
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Nicholas S. Frost
7 Avenida Vista Grande #325
Santa Fe, NM  87508
nickf at nickorama.com
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