Centos – running mongodb with openvz

centosmediatemplemongodbopenvz

Has anyone had any luck getting mongodb working on openvz?

Was looking for some tips, tricks, hacks to get it from running out of memory

I am running on centos on media temple and the servers just run out of memory really quickly

I know its not recommended to run mongo on openvz but wondering if anyone knows how to make it work

Thanks

Best Answer

One of the key problems with OpenVZ from the guest OS's point of view is that it is possible for the host OS to overcommit its RAM, allowing a VM to temporarily use more the amount it is guaranteed. It is an issue for processes that use a lot of RAM and assume that once they are allocated it they can just keep it so do for caching reasons.

IIRC the way Mongo uses files and memory doesn't allow for a method of limiting how much memory it might try to use. If it see fit it will try take all the memory that is available, less a bit for the rest of the OS, and under OpenVZ that will be the maximum amount allocated not your guaranteed amount.

I have seen talk of a way to limit it under Windows 2008r2 (see here) but that doesn't help you with CentOS running under OpenVZ. One option you could try is setting the VMs' maximum RAM allocation to be the same as their guaranteed allocation (or move to a virtualisation solution that does not all memory over-commit) - this should discourage Mongo and the Kernel from creating an unstable OOM situation.

Related Solutions

Linux – How to tune system settings for mongoDB on Linux

I Think your case with 4 Gigs of RAM is pretty good covered here for Neo4j which also uses Memory Mapped IO and thus the same tuning measures should apply:

http://docs.neo4j.org/chunked/stable/linux-performance-guide.html

So from reading this I think your

vm.dirty_ratio=5
vm.dirty_background_ratio=5

are rather too low than to high.

MongoDB getting OOM killed

OOM killer is not a way anyone manages memory; it is Linux kernels way to handle fatal failure in last hope to avoid system lockup!

What you should do is:

make sure you have enough swap. If you are sure, still add more.
implement resource limits! At LEAST for applications you expect that will use memory (and even more so if you don't expect them to - those ones usually end up being problematic). See ulimit -v (or limit addressspace) commands in your shell and put it before application startup in its init script. You should also limit other stuff (like number of processes -u, etc)... That way, application will get ENOMEM error when there is not enough memory, instead of kernel giving them non-existent memory and afterwards going berserk killing everything around!
tell the kernel not to overcommit memory. You could do:

echo "0" > /proc/sys/vm/overcommit_memory

or even better (depending on your amount of swap space)

echo "2" > /proc/sys/vm/overcommit_memory; echo "80" > /proc/sys/vm/overcommit_ratio

See Turning off overcommit for more info on that.

That would instruct kernel to be more carefull when giving applications memory it doesn't really have (similarity to worlds global economic crisis is striking)
as a last dich resort, if everything on your system except MangoDB is expendable (but please fix two points above first!) you can make lower the chances of it being killed (or even making sure it won't be killed - even if alternative is hangup machine with nothing working) by tuning /proc/$pid/oom_score_adj and/or /proc/$pid/oom_score.

echo "-1000" > /proc/`pidof mangod`/oom_score_adj

See Taming the OOM killer for more info on that subject.

Best Answer

Related Solutions

Linux – How to tune system settings for mongoDB on Linux

MongoDB getting OOM killed

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