[mythtv-users] anyone using software RAID 5 for 1 terabyte +

[chris at cpr.homelinux.net]

On Sun, Aug 06, 2006 at 05:59:26PM -0400, Richard Freeman wrote:
> As far as I can tell that doesn't support anything along these lines
> that can't be done with md and lvm.  If I have 5x250GB drives in a 1TB
> RAID-5 with one redundant drive, and I want to add a single 250GB drive
> to the array to obtain 1.25TB of storage, this is not trivial to
> accomplish with any of the tools (actually, this can be done with md
> offline at some risk in the event of a power failure).

There is new kernel+mdadm code that allows you to add active drives 
to a RAID-5 array (older versions could only resize the drives or 
add spares).  From what I've read so far, it can even do the 
addition while the system is up and running.  When you tell mdadm 
to add another drive it momentarily locks the entire array (writes 
are spooled) so it can move some metadata and enable crash 
recovery, and then does the rest of the rebuilding with the 
filesystem unlocked much like it does rebuilds of existing arrays.

> What I would like to see is a solution for mixtures of drive sizes.

You can impliment RAID-5 using logical volumes instead of physical 
drives or partitions.  If you have an old 200Gb and a 60Gb you 
could merge them to create a 260Gb device and then add it to the 
array.  That logical device would be twice as likely to fail as the 
others, of course.

> Suppose I have 4 120GB drives in RAID-5 and I want to add a 250GB drive
> and a 60GB drive to the array.  In theory this can be accomplished by
> chopping everything into 10GB blocks

A better idea would be to donate the 60Gb drive to a charity and 
split the 250Gb drive into two partitions, one of which is added to 
the RAID-5 and the other of which is used in some other way.  For 
example, on my system the drives are not all the same sizes, so the 
two largest drives each provide a partition to the RAID-5 array and 
a partition to a RAID-0 array.  The remaining drives supply the 
RAID-5 only.  I end up with two arrays that can both survive the loss of
any single drive.

> To me the biggest limitation of RAID-5/6/whatever is the fact that the
> drives all have to be the same size.

No.  The rule is that the array will only use the same amount of 
space from each drive.  The drives can be any size you want.  When 
one of my 160Gb drives died last week I ended up buying a 320Gb 
replacement.  I created a 160Gb partition and the RAID-5 rebuilt 
just fine.

> The other pain with most RAID setups is the fact that it is a pain to
> implement them using existing drives full of data.

I guess that depends on your definition of pain.  As a geek, I saw 
implimenting RAID-5 in-situ as an exciting challenge.  I wouldn't 
want to do it on a customer's machine, but doing it on my home box 
brought a fair sense of accomplishment.  It's even easier now that 
RAID-5 can add drives to an existing array.

Hint: when creating the array, use the UUID instead of device names 
in the mdadm.conf file so that you can swap drives or move 
partitions and have them automatically found and activated.