Re: Session and Multi Server Architecture

Stut wrote: > Per Jessen wrote: >> mike wrote: >> >>>> Check out persistency in LVS for instance: >>>> >>>> http://www.linuxvirtualserver.org/docs/persistence.html >>> i know persistence handling is an option in LVS, but i haven't seen >>> the need to use it. i use LVS right now without even bothering with >>> any of that. >> >> Because you've chosen another option - memcached presumably - which >> is more expensive over all. (IMHO). > > Based on what? I'm currently looking at replacing a file-based cache > with memcached and I'm interested in all justified opinions. You might not have much use for mine then :-) If I compare plain session-persistency (session being a client to server relationship, not $_SESSION) with memcacheds copying objects around across many machines, common sense tells me that the latter will use a lot more resources. Both on the network and on each machine. With memcached, and 1000 clients for a cluster of 50 machines, each using up 1Mb of $_SESSION space, that is 1000Mb per machine, 50Gb in total. With session-persistency, you have 50 machines with 1000/50 clients each, making 20Mb per machine. And virtually no session-related inter-machine network traffic. And your machines are not busy with keeping 980 objects needlessly up-to-date. Of course, processing power, network capacity and memory are all very cheap these days, so it's easy to put on the Microsoft hat and be wasteful. >> On the next request, LVS will know not to try that server, and the >> user will move to another one. Obviously the session-context will >> die, but is that really a big deal? How often does one of your >> servers die? Sure, if you've got 10,000 in a cluster, you'll have >> fans and harddisks pop every so often, but I have my doubts about >> memcached scaling to that level (please correct me if I'm wrong here, >> I have _no_ experience with memcached). > > Facebook. Digg. LiveJournal. That enough scalability proof for ya? If > not there are plenty more where those came from. It wasn't really the scalability I was interested in here, but I'd be interested to hear some numbers if you have any. My point with the 10,000 machines was - with that many components, even the 100,000 hours MTBF of a regular harddisk will mean one will break every ten hours (statistically speaking). Add fan- and other failures to that, and you'll have sessions dying quite frequently. Which _could_ be a significant problem, and maybe enough to make you want to use memcached for object replication. (there might also be a restriction in the number of concurrent clients handled by LVS session persistency). However, with a much smaller cluster, say 50 or 100 machines, hardware failures will be a lot less frequent, and having the odd session die once a week might not be a problem. Which is why I think LVS session persistency is perfectly fine. /Per Jessen, Zürich