RE: Software based SATA RAID-5 expandable arrays?

["Daniel Korstad" <dan@korstad.net>]

  
That was true up to kernel 2.6.21 and 2.6 mdadm where support for RAID 6 reshape arrived.
 
I have reshaped (added additional drives) to my RAID 6 twice now with no problems in the past few months.
 
You mentioned that as the only disadvantage.  There are other things to consider.  The overhead for parity of course.  You can't have a RAID 6 with only three drives unless you build it with a missing drive and run degraded.  Also (my opinion) it might not worth the overhead with only 4 drives, unless you plan to reshape (add drives) down the road.  When you have an array with several drives, than it is more advantages as the percentage of disk space lost to parity goes down [((2/N)*100) where N is the number of drives in the array] so your storage efficiency increases ((Number of Drives -2)/Number of Drives).  And with more drives the statistics of getting hit with a bit error after you lose a drive and you are trying to rebuild increases.
 
Also, there is a very slight performance drop for write speeds on RAID6 since you are calculating p and q parity. 

But for what I use my system for, family digital photos, file storage and media server I mostly read data and not bothered with slight performance hit in write.
 
I have been using RAID6 with 10 disk for over a year and it has saved me at least once.  
 
As far as converting the RAID6 to RAID5 or RAID4...  Never had a need to do this, but no probably not.
 
Dan.

 

----- Inline Message Follows -----
To: Daniel Korstad ; Michael 
Cc: linux-raid@vger.kernel.org
From: jahammonds prost
Subject: Re: Software based SATA RAID-5 expandable arrays?



> Why do I use RAID6?  For the extra redundancy 

I've been thinking about RAID6 too, having been bitten a couple of times.... the only disadvantage that I can see at the moment is that you can't convert and grow it... ie... I can't convert from a 4 drive RAID5 array to a 5 drive RAID6 one when I add an additional drive... I also don't think that you can grow a RAID6 array at the moment - I'd want to add additional drives over a few months as they come on sale.... Or am I wrong on both counts?


Graham