[sclug] Mirror disks across machines like distributed RAID?

Tom Carbert-Allen tom at randominter.net
Wed Jan 6 19:49:31 UTC 2010

On 06/01/2010 6:34, Dickon Hood wrote:
> How does DRBD help you in this situation?  You can't mount the same
> filesystem on two hosts, remember.
Erm, you can and we do; OCFS2 was created exactly for that reason. This 
is how Oracle redundant application cluster works (well they still 
support ASM too but that is basically the same thing but Oracle apps 
only instead of a Linux file system).
Two nodes sharing one cluster-ready file system is the new replication.
That layer then sits on a shared medium, be that either a SAN or SAN 
array or DRBD device.

As an aside, I have tested Oracle RAC extensively for performance on 
OCFS2 on DRBD local disks compared to FC SAN's and found not only does 
DRBD save you money, space, power and maintenance. It is also much 
faster for the same amount of disks. In some test the IOPS went up 40% 
on the exact same disks!
> I'd probably do something with NFS or the like.
Which is served from what? A single point of failure server? A expensive 
EMC array which is basically doing exactly what DRBD does but for 
infinite times more cost? The question is, are your cluster of front end 
servers there just for capacity or redundancy, because for redundancy 
your disk system has to be redundant too otherwise there is no point.

DRBD is about ensuring your NFS server or you OCFS2 servers underlying 
block device stays on line by way of having two copies of the data which 
can both continue to work should the other fail. Just like the OP 
subjest line, it's one step up from software raid, it's software raid 
with 100% separate hardware for each part of the mirror. It also helps 
that this then means each half of the mirror is local to each machine, 
and the CPU load is almost non-existent and the speed of magnetic disks 
so you can integrate your redundant storage array into a pair of your 
next layer nodes and remove the round trip time on reads.

The bottom line is we should all rejoice that what you used to have to 
pay EMC $xxxxx  for is now in our favourite mainline kernel totally for 
free with no extra hardware! Just like you couldn't have RAID before 
unless you bought a card which cost as much as your motherboard, but now 
anyone can have a nice mirror for just the price of the drives.
Not everyone will use it, but those of us that do are very happy and 
want to let everyone know they can do it too for no cost!!!


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