[Linux-cluster] General GFS Advice?

Matt Brookover mbrookov at mines.edu
Mon Dec 12 21:41:23 UTC 2005

On Mon, 2005-12-12 at 12:27, D Canfield wrote:

> I'm just looking for a bit of general advice about GFS... We're 
> basically just looking to use it as a SAN-based replacement for NFS.  
> We've got a handful of servers that need constant read/write access to 
> our users' home directories (Samba PDC/BDC, web server, network terminal 
> servers, etc.), and we thought GFS might be a good replacement from a 
> performance and security standpoint, let alone removing the SPOF of our 
> main NFS/file server.  Another place we're thinking of using it is 
> underneath our mail servers, so that as we grow, SMTP deliveries (and 
> virus scanning) can happen on one machine while IMAP/POP connections can 
> be served through another.

I am working on using GFS for home directories, smtp, imap, pop and
web.  So far things are going pretty good.  (see my posting from this
morning for one down side)  At some point we may add Samba and other
services.  The current goal is to move mail and web servers to a set of
inexpensive, highly available, and scalable servers.

Be carefull with performance, GFS adds a lot of overhead and can be very
slow when dealing with lots of small file creates or deletes.  After I
did a lot of testing, we decided that the overhead was not a problem for
our application.

We hacked up some crude tools to push test mail messages and a thing to
read and verify the messages.  At one point the cluster was running 600
imap connections and had nearly 1500 inbound mail messages/minute.  The
simulated users were reading a message every 10 seconds, after reading
the mail box, would go back and delete a message every 2 seconds then
expunge the mail box.  There were 30 mail readers running on 20 linux
workstations.  The mail push consisted of 10 process on 5 workstations
that would generate a random message then send it by SMTP to cluster. 
Without any delay, the push would send around 3300 messages/minute and
swamp the cluster.  After some trial and error, I found that a delay of
1.7 seconds between messages would slow the push rate to around 1400
messages/minute.  The big bottle neck is openldap.  Its load goes up
serving sendmail aliases.  I expect our first upgrade will be to put in
more LDAP servers to spread the load.

> Unfortunately, even at academic prices, Red Hat wants more per single 
> GFS node than I'm paying for twenty AS licenses, so I've been heading 
> down this road by building from the SRPMS.  I mostly have a 2-node test 
> cluster built under RHEL4, but a number of things have me a little bit 
> hesitant to move forward, so I'm wondering if some folks can offer some 
> advice. 

I used the source RPM also, for the same reasons.  I would love to be
able to purchase support, but the funding is not there.  I would suggest
that you test carefully, and move slowly.

> For starters, is my intended use even appropriate for GFS?  It does seem 
> as though I'm looking to put an awful lot of overhead (with the cluster 
> management suite) onto these boxes just to eliminate a SPOF. 

I think this debate is ongoing for any body that is looking at a SAN or
cluster.  Once I factored in GFS, LDAP, Kerberos, load balancers, a SAN,
etc, this has turned into the most complex system I have ever built for
an employer.  The 2 times we have had problems, one of the other servers
took over and the traffic went through.  We are still in test mode, but
expect to put our first cluster in production on January 7th.

> Another concern is that this list seems to have a lot more questions 
> posted than answers.  Are folks running into situations where 
> filesystems are hopelessly corrupted or that they've been unable to 
> recover from?  That's the impression I feel like I'm getting, but I 
> suppose a newbie to Linux in general could get the same impression from 
> reading the fedora lists out of context.    The last thing I want to do 
> is put something into production and then have unexplained fencing 
> occurences or filesystem errors.

I have been working with GFS for over a year now, on both test and
soon-to-be production servers.  In general, I think GFS works well.

EXT3, JFS, etc are more stable.  The down side is that if the server
that holds the EXT3 is down, then the applications are down.  It is nice
to be able to take a server out of production, fix it, make changes, etc
and the users not know. 

> Finally, Red Hat sales is laying it on pretty heavy that the reason the 
> GFS pricing is so high is because it's nearly impossible to install it 
> yourself.  That was particularly true before GFS landed in Fedora.  Now 
> the claim is just that it's very difficult to manage without a support 
> contract.  Is this just marketing, or does GFS really turn out to be a 
> nightmare to maintain?

While testing, I have built on Fedora core 3, Suse 9.1, and RedHat
Enterprise 3.  I have built from the CVS tree, and SRPMS, and never had
much trouble getting GFS up and running.  I did have to write a fencing
module to work with a Cisco switch (not difficult, it is just a perl
script that does some SNMP calls).  The SAN is ISCSI based and the only
place to fence was in the switch.  Of course, I have been hacking Unix
boxes for 20 years now and using Linux in development and production for
10 years.  If you background is Windows or VMS, you would have to work
at it.

> Any insights people could provide would be appreciated.

Move slow, plan carefully, and test everything.  Of course, this is my
standard advice any time you are doing something new. 

mbrookov at mines.edu

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