[Gluster-users] gluster ha/replication/disaster recover(dr translator) wish list

[Prabhu Ramachandran]

On 01/27/09 02:21, Keith Freedman wrote:
> At 10:36 AM 1/26/2009, Prabhu Ramachandran wrote:
>> Are you saying I can't use the local files on machine A (even if I am
>> not touching any files on B) when the network is down even though all I
>> am doing is reading and perhaps writing locally on A?  That could be a
>> bit of a problem in my case since I usually keep any local builds on the
>> partitions I'd like to share.  There could be other problems when one
>> machine is down for maintenance for example (or the disk crashes).
> 
> I'm not saying that.  you can use the files on the local machine.  and 
> it will, it'll just have to wait for network timeouts to decode that 
> machine B  is down before it continues.

OK that is consistent with what I had in mind.

> The problem would be if the network between A and B is down.  you update 
> files on A (say run a build), and simultaneously update files on B (run 
> a build there too), then the network comes back up, the same files might 
> have changed on both A & B at the same time, and this would cause a 
> split-brain.
> 
> but if you always only update on A, then it doesn't matter.  when the 
> network is down, B will have local access to the version of files that 
> were correct as of the last time they were in sync, but it will still 
> serve them.

I think this should work fine for me.

>> The attachment is scrubbed which makes it a pain to get from there.  I
>> enclose the relevant parts below.  Many thanks.
> 
> I dont see any problems with your config.
> other than, if your network connection is very sporadic, then you'll be 
> caught often by waiting for timeouts which will make things seem slower.

Network connection isn't usually a problem but when the network does go 
it could be gone for a while.  In the worst case could I temporarily 
disable the replicate/AFR feature?

> What you seem to want is for gluster to serve local files instantly, but 
> it can't because in HA mode, it needs to know that either the replica is 
> down, or that is has the most current version.  if your network is 
> spotty, then it will constanly be waiting to decide that the network is 
> down before continuing on, that's likely the concern that was raised 
> earlier.
> 
> but if it's more likely that the network will be down for a while then 
> up for a while, then it's not a big deal and you should be just fine.

I could use this option:

read-subvolume (default: none)

This might actually be the best solution in this case.  I could have 
each client only read files from the local subvolume and sync the writes 
when that happens.  There will be a problem with any writes though but I 
might just have to live with that or disable AFR for a while.

Thanks for all the clarifications and your patience!

cheers,
prabhu