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Re: [Linux-cluster] dlm and IO speed problem <er, might wanna get a coffee first ; )>





On Mon, Apr 7, 2008 at 9:36 PM, christopher barry <Christopher Barry qlogic com> wrote:
Hi everyone,

I have a couple of questions about the tuning the dlm and gfs that
hopefully someone can help me with. 
 

There are lots to say about this configuration.. It is not a simple tuning issue.



my setup:
6 rh4.5 nodes, gfs1 v6.1, behind redundant LVS directors. I know it's
not new stuff, but corporate standards dictated the rev of rhat.
 

Putting a load balancer in front of cluster filesystem is tricky to get it right (to say the least). This is particularly true between GFS and LVS, mostly because LVS is a general purpose load balancer that is difficult to tune to work with the existing GFS locking overhead.


The cluster is a developer build cluster, where developers login, and
are balanced across nodes and edit and compile code. They can access via
vnc, XDMCP, ssh and telnet, and nodes external to the cluster can mount
the gfs home via nfs, balanced through the director. Their homes are on
the gfs, and accessible on all nodes.
 

Direct login into GFS nodes (via vnc, ssh, telnet, etc) is ok but nfs client access in this setup will have locking issues. It is *not* only a performance issue. It is *also* a function issue - that is, before 2.6.19 Linux kernel, NLM locking (used by NFS client) doesn't get propagated into clustered NFS servers. You'll have file corruption if different NFS clients do file lockings and expect the lockings can be honored across different clustered NFS servers. In general, people needs to think *very* carefully to put a load balancer before a group of linux NFS servers using any before-2.6.19 kernel. It is not going to work if there are multiple clients that invoke either posix locks and/or flocks on files that are expected to get accessed across different linux NFS servers on top  *any* cluster filesystem (not only GFS). .
 


I'm noticing huge differences in compile times - or any home file access
really - when doing stuff in the same home directory on the gfs on
different nodes. For instance, the same compile on one node is ~12
minutes - on another it's 18 minutes or more (not running concurrently).
I'm also seeing weird random pauses in writes, like saving a file in vi,
what would normally take less than a second, may take up to 10 seconds.

* From reading, I see that the first node to access a directory will be
the lock master for that directory. How long is that node the master? If
the user is no longer 'on' that node, is it still the master? If
continued accesses are remote, will the master state migrate to the node
that is primarily accessing it?


Cluster locking is expensive. As the result, GFS caches its glocks and there is an one-to-one correspondence between GFS glock and DLM locks. Even an user is no longer "on" that node, the lock stays on that node unless:

1. some other node requests an exclusive access of this lock (file write); or
2. the node has memory pressure that kicks off linux virtual memory manager to reclaim idle filesystem structures (inode, dentries, etc); or
3. abnormal events such as crash, umount, etc.

Check out: ,
http://open-sharedroot.org/Members/marc/blog/blog-on-gfs/glock-trimming-patch/?searchterm=gfs
for details.


I've set LVS persistence for ssh and
telnet for 5 minutes, to allow multiple xterms fired up in a script to
land on the same node, but new ones later will land on a different node
- by design really. Do I need to make this persistence way longer to
keep people only on the first node they hit? That kind of horks my load
balancing design if so. How can I see which node is master for which
directories? Is there a table I can read somehow?
 

You did the right thing here (by making the connection persistence). There is a gfs glock dump command that can print out all the lock info (name, owner, etc) but I really don't want to recommend it - since automating this process is not trivial and there is no way to do this by hand, i.e. manually.



* I've bumped the wake times for gfs_scand and gfs_inoded to 30 secs, I
mount noatime,noquota,nodiratime, and David Teigland recommended I set
dlm_dropcount to '0' today on irc, which I did, and I see an improvement
in speed on the node that appears to be master for say 'find' command
runs on the second and subsequent runs of the command if I restart them
immediately, but on the other nodes the speed is awful - worse than nfs
would be. On the first run of a find, or If I wait >10 seconds to start
another run after the last run completes, the time to run is
unbelievably slower than the same command on a standalone box with ext3.
e.g. <9 secs on the standalone, compared to 46 secs on the cluster - on
a different node it can take over 2 minutes! Yet an immediate re-run on
the cluster, on what I think must be the master is sub-second. How can I
speed up the first access time, and how can I keep the speed up similar
to immediate subsequent runs. I've got a ton of memory - I just do not
know which knobs to turn.

The more memory you have, the more gfs locks (and their associated gfs file structures) will be cached in the node. It, in turns, will make both dlm and gfs lock queries take longer. The glock_purge (on RHEL 4.6, not on RHEL 4.5) should be able to help but its effects will be limited if you ping-pong the locks quickly between different GFS nodes. Try to play around with this tunable (start with 20%) to see how it goes (but please reset gfs_scand and gfs_inoded back to their defaults while you are experimenting glock_purge).

So assume this is a build-compile cluster, implying large amount of small files come and go, The tricks I can think of:

1. glock_purge ~ 20%
2. glock_inode shorter than default (not longer)
3. persistent LVS session if all possible


Am I expecting too much from gfs? Did I oversell it when I literally
fought to use it rather than nfs off the NetApp filer, insisting that
the performance of gfs smoked nfs? Or, more likely, do I just not
understand how to optimize it fully for my application?


GFS1 is very good on large sequential IO (such as vedio-on-demand) but works poorly in the environment you try to setup. However, I'm in an awkward position to do further comments  I'll stop here.

-- Wendy



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