Re: md with shared disks

[Stan Hoeppner <stan@hardwarefreak.com>]

With DRBD and GFS2 it is true active/active at the block level.  You
just lose half your disk capacity due to the host-to-host mirroring.
Whether your upper layers are active/active is another story.  E.g.
getting NFS server/client to do seamless automatic path failover is
still a shaky proposition AIUI.

You mention multipath.  If you plan to use iSCSI multipath for the i7
servers you need to make sure each LUN you export has the same WWID on
both cluster nodes.

Stan



On 11/13/2014 07:14 AM, Anton Ekermans wrote:
> Thank you very much for your clear response.
> The purpose of this hardware is to primarily host ample VM storage for
> the 2 nodes itself and 3 other i7 PC/servers.
> The HA was hoped to be achieved as active/active with both nodes sharing
> the same disks and non-cluster servers(i7) having multi-path to these
> two nodes. This is advertised as HA active/active in storage software
> such as Nexenta using RSF-1. However upon closer inspection, their
> active/active means both nodes share some data and the other can take
> over. So for me, in essence it is "active/passive + passive/active" and
> not truly "active/active". We will try to config this way to get quasi
> active/active for best performance with kind-of high-availability. Seems
> the shared disks is not the problem, but combining them on a cluster is.
> 
> Thank you again
> 
> Best regards
> Untitled Document
> 
> Anton Ekermans
> 
>> It's not possible to do what you mention as md is not cluster aware.  It
>> will break, badly.  What most people do in such cases in create two md
>> arrays, one controlled by each host, and mirror them with DRBD, then put
>> OCFS/GFS atop DRBD.  You lose half your capacity doing this, but it's
>> the only way to do it and have all disks active.  Of course you lose
>> half your bandwidth as well.  This is a high availability solution, not
>> high performance.
>>
>> You bought this hardware to do something.  And that something wasn't
>> simply making two hosts in one box use all the disks in the box.  What
>> is the workload you plan to run on this hardware?  The workload dictates
>> the needed hardware architecture, not the other way around.  If you want
>> high availability this hardware will work using the stack architecture
>> above, and work well.  If you need high performance shared filesystem
>> access between both nodes you need an external SAS/FC RAID array and a
>> cluster FS.  In either case you're using a cluster FS which means high
>> file throughput but low metadata throughgput.
>>
>> If it's high performance you need, an option is to submit patches to
>> make md cluster aware.  Another is the LSI clustering RAID controller
>> kit for internal drives.  Don't know anything about it other than it is
>> available and apparently works with RHEL and SUSE.  Seems suitable for
>> what you express as your need.
>>
>> http://www.lsi.com/products/shared-das/pages/syncro-cs-9271-8i.aspx#tab/tab2
>>
>>
>>
>> Cheers,
>> Stan
>