New server at the shop

It has been a long time since I’ve written anything, I know. I’ve been so busy recently with several different things; I’ve wanted to write about all of it but I just haven’t had the time. Maybe I’ll cover a little bit more of it soon but right now I wanted to talk about an interesting problem we had with the new server at the computer shop.

On the hardware side it’s a Dell PowerEdge 1950 with a quad-core Xeon 2.33GHz CPU, 4 GB RAM, and a mirrored 500 GB RAID array. It’s not what you’d call cutting edge in the data-center but it’s a far cry from the dorky little single-core Pentium 4 thing we had been using.

Software-wise it’s running Ubuntu 10.04 Server which is taking care of our file-server needs along with housing our squid caching proxy and PXE boot environments. Since this server is equipped with dual gigabit LAN we bit the bullet and bought a couple of gigabit switches as well, which has really made a difference in Quickbooks’ performance alone.

For various reasons (not least of which is Quickbooks) all of our workstations in the shop but mine are running Windows which means file sharing must be done over SMB using Samba. I set everything up as usual with mostly defaults in my smb.conf. The shares were added as usershares with full write access by everyone for the sake of simplicity; that shouldn’t be a problem since iptables is blocking all traffic originating from the WAN port.

Since everyone was so excited about the gigabit stuff, my boss fired up a couple of file transfers and began listening to mp3 files over SMB just to see what kind of performance we could get. The funny thing was that every so often the music would just hang for no reason for 20-30 seconds at a time. During the hang his computer wouldn’t be able to browse the file shares at all and all file transfers would hang as well. I checked the system logs on his computer and dmesg on the server and neither mentioned anything related to the problem. Finally I found the Samba session logs (these are in /var/log/samba on Ubuntu) which are unique per client and saw lines in it like this:

"Unable to connect to CUPS server localhost:631 - Connection timed out"

Timed out? Yeah, that sounds like it. A bit more investigation led me to realize the timing of the pauses and the errors seemed to correlate. Searching for that error on the Internet was very difficult, though; it seemed like everybody with a Samba client log posted on the web had that error and nearly all the posts I found we’re asking about unrelated issues.

Disgusted with searching for a bit, I tried the obvious solution of commenting out all printer related lines in smb.conf. No luck…apparently printing support is enabled by default even if it isn’t configured. Back to searching.

Finally I found one guy who was complaining about a big lag during log-in on password protected shares and was getting that same CUPS error. He figured out how to get Samba to stop trying to connect to CUPS using these configuration lines. He admits that they might not all be needed, but it doesn’t hurt my feelings to smash a bug a little harder than necessary:

load printers = no
printing = bsd
printcap name = /dev/null
disable spoolss = yes

Around the same time I also found that many people were saying that this particular configuration tweak was supposed to give better throughput when serving Windows clients:

socket options = TCP_NODELAY

I shamelessly applied both fixes at once.  At the time I just really needed it to work (we’d already moved our Quickbooks company file) and didn’t care too much which one fixed it so long as it was fixed.  And it was.

No one really mentioned the socket options tweak affecting any sort of hangs or delays so I’m certain that disabling print services in Samba was the real kicker.  Apparently there are very few people who set up Samba on a server without CUPS, but we have no need for a print server like that.  We don’t have a big copier or anything; our only printer is a small black and white laser at the front desk which is 30 feet and several walls away from our server.

So we have another problem solved and I feel pretty good about it. It’s nice to find the solution to a tough problem and I thank the good Lord for helping me.

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