RE: [Xen-users] Xen SAN Questions

["Tait Clarridge" <Tait.Clarridge@xxxxxxxxxxxx>]

Thanks again for the reply.

>> The reason for this was so that I could have two Xen hosts using  
>> their local storage but to also have it replicating between servers  
>> for backup purposes. Maybe I was going about this the wrong way, but  
>> I wanted to have the ability to use the large storage pool (created  
>> by the cluster) as a platform for storing VMs as well as have it  
>> back everything up on two separate locations of disk.

>I think DRDB will work here (never used it personally), but I think  
>I'd create a DRDB device for each VM and replicate each one separately  
>so you could have a primary on the remote side running a vm that can  
>be started locally if the remote site blows up and vice-versa.  You  
>don't need the added overhead of a cluster filesystem for this.  DRDB  
>will certainly add overhead for replicating writes, but this is highly  
>tweakable (consider you're replication write rate, for example, where  
>maybe you're writing to the disk at 100MB/s but only replicating at  
>10KB/s, thus saving a ton of i/o).

Alright, however we do already have the images set as raw files and converting 
some of the systems (which are running in production) to a DRBD devices might 
prove difficult. It's not a question of whether we can do that, but it is a 
good idea (for eliminating the need for clustering). I will definitely have a 
look in the replication write rate, but wouldn't a lower rate mean that some of 
the more I/O intensive VMs will lose data in the event of something going wrong?

I am kind of a newb when it comes to replication :)

And since everything is in image files, what do you think about exporting the 
DRBD'd storage as NFS for both machines to read and write? Risky? Inefficient? 

>> To clarify, the disks themselves will be a RAID-5 local to each  
>> machine (1 array per machine, 2 in total) with DRBD running between  
>> to sort of RAID-1 them over the network. Does that help? I want to  
>> take the local RAID from both machines and turn it into a SAN.

>Cool, that's fine, but don't say "SAN" here, they're different.

My mistake, sometimes I confuse terms.

Best,
Tait


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