Linux Print System at Cisco Systems, Inc.
Every time a new printer was installed, not only did I have to create a queue on my UNIX print servers, but the Desktop Technology Group also had to create a separate queue on their NT servers. To make matters worse, the drivers on the PC had to be carefully matched with the drivers used under NT.
I investigated Samba and was extremely impressed with its ability to provide the same services to PCs using the same protocols NT used. Essentially, one could make a UNIX machine pretend to be an NT server. Samba is also extremely configurable—so much that it is easy to be confused by the enormous array of choices.
Expecting rejection and a passionate argument of NT versus UNIX, I approached the Desktop Technology Group with the idea of taking over PC printing. Their response was, “Really? You'll take over setting up and managing these obnoxious printers for us? Hey, it's all yours!”
The Samba protocol (SMB), however, has a severe limitation: the browsing (which allows the user to get a list of available printers) is done using a single UDP packet, and so is limited to about 8KB worth of printer names and descriptions. In our environment, this translated to about 50 printer names per server. However, I found Samba had the ability to use different configuration files according to the name the PC thinks the server is called. For example, suppose that for one physical server, I register the two pseudo-server names pserver1 and pserver2 in Microsoft's version of DNS, WINS. The PC will see two servers, pserver1 and pserver2, both referring to the same physical Samba server. The Samba server will pick a different configuration file and serve a different set of printers according to which pseudo-server the PC thinks it is talking to. This allows us to effectively break the 8KB barrier by separating printers into smaller groups.
The printing itself is simple. The Samba configuration files specify a program or script to run when a print job is received. Usually, this is a simple lpr command. The PC queue display is done by converting the output of the lpq command.
The print servers could, of course, talk to all the different printer protocols. Add to this the Samba capability of receiving jobs from any PC, and now any PC can send a job to any printer—even an AppleTalk printer.
I could have just associated the pseudo-server names with the loccodes; however, I found this lacked the flexibility I needed. Since the PC printer path (in the form of “\pseudo-server\printer-name”) was fixed in people's PCs, I found that if I moved a printer from one loccode to another, people would have to re-install the printer on their PCs. This caused a problem when I was making administrative changes like splitting a loccode up or joining two together; in this case, I wanted to be able to make these changes without involving the user.
The only way to do this was to associate the pseudo-server name not with the loccode, but directly to another grouping: the “service group” or “sgroup”. I could associate a loccode to one or more of these sgroups. Conversely, each sgroup could have multiple loccodes associated with it. To use database parlance, loccodes and sgroups have a many-to-many relationship. The sgroup concept allows me to split, join or move loccodes without changing the client PCs.
For more information on sgroups and loccodes as well as an example of how they are used, see the “Loccodes and Sgroups” sidebar.
Samba, and all these damn sgroups, were beginning to put a sizable load on the print servers. We had upgraded the print servers at our headquarters in San Jose to two Sun SPARC 20s; however, the growth in PC usage was soon stressing even these two. To make matters worse, Windows 95 has a very short time-out period when asking for a listing of the printer queue (less than three seconds). If it times out, the printer is marked on the PC as “off-line”, and the user is required to go into the settings and put the printer back on-line. This feature was generating too many calls.
I now introduced a couple of Red Hat Linux machines as Samba print servers. The servers were HP XM4s: 120MHz Pentiums, with 32MB RAM and 1GB hard disk. These were the same PCs that were used as the standard desktop machines at Cisco.
I had not ported the entire printing system from SunOS to Linux, so I could not use them as final print servers—that task was left to one of the two SPARC 20s. The Linux print servers would receive the print jobs from the PCs using Samba and then forward them to the SPARC print servers. This was possible due to the sgroup and loccode system I explained earlier.
|August 2014 Issue of Linux Journal: Programming||Aug 01, 2014|
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|Open-Source Space||Jul 31, 2014|
|Silicon Mechanics Gives Back||Jul 30, 2014|
|Reglue: Opening Up the World to Deserving Kids, One Linux Computer at a Time||Jul 29, 2014|
|diff -u: What's New in Kernel Development||Jul 23, 2014|
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