[stgt] Quick Failover & Scalability questions

Mark Lehrer mark at knm.org
Wed Oct 13 18:26:25 CEST 2010

I have a few tgtd servers running and they are doing very well.  I would 
like to add a 2nd server as a backup so I have one question.  Assuming that 
the block device mirroring and IP/Mac failover are perfect ( kind of like 
assuming a frictionless surface in physics :) ), how well does stgt fail 
over to a 2nd server?

The simplest would be for the clients to reconnect to the new server and 
re-establish communications.  However, how painful would it be for the new 
server to keep the same sockets open for a truly seamless failover?  Again, 
I am only concerned about the tgtd internal states at this point - assume 
that the block device mirroring as well as the 
keepalived/heartbeat/iptables/fencing/etc issues are handled already (though 
there would obviously be a good bit of integration work there!).

My 2nd question - I am starting to play with things like ionice and 
scalability.  One critical aspect of scalability is limiting the first 
connection's performance so it doesn't degrade so quickly as more clients 
come online.  It appears that ionice will probably not have the features I 
need; is this on the tgtd roadmap?  Even something as basic as a generic "io 
operations per second" would be a good start.

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