Capacity

Re: Capacity

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At 4:09 PM -0600 11/5/99, Jerry Pasker-Systems Admin. wrote:
>When I see a lot of "Looking up host..." in my browser window, I know that
>my DNS server is the delay. I'm not heavy into web site development. 99%
>of my business is as a dial up ISP, and it's pretty competitive out there.
>At the bottom of the navigator window, people don't want to see "Looking up
>host www.XYZ.com" they want to see "Host Contacted... Transferring data"
>and "Transfer complete".
>I need to keep latency from delayed DNS look ups down to a minimum.

Does your network provider have DNS servers? Do they get even more traffic than yours? If so, they probably have a good-sized cache of records. You can use the forwarding option (in the Server Preferences dialog) to take advantage of this cache, rather than having your server look everything up itself.

>Let's see if I got this.......
>It seems that as long as QDNS Pro has enough RAM to hold all the zone
>files, more RAM is unnessasary if most of the quirres are inbound from
>other DNS servers. This would be the situation for someone hosting
>multiple moderate, or a few busy websites, and few clinet machines using
>the DNS server.

Absolutely correct.

> More RAM and CPU time would definately be required if the
>DNS server was primarily used by a large number of client machines to do
>DNS lookups. This would be the situation for an ISP with a moderate sized
>client base, and modem ports.

Not *required*, but desirable. If QDNS runs out of free memory, it will purge the least-used entries in its cache (not counting local domains, of course) until it has sufficient room. It has a very good cache-management algorithm for this situation.

>More RAM is better, but too much RAM is bad, unless you need it to hold
>zone files.

Basically. Defining "too much RAM" is the tricky part.

>The trick is to have enough RAM to hold the cachced entries, enough CPU to
>parse that cache, but not so much RAM that the CPU can't parse the DNS
>cache fast enough.
>
>Do I understand this right?

Yes. Keep in mind that your 6220 is an extreme case. On a decent machine, you should be able to give QDNS 40 MB of memory and see it use it all, without slowing itself down.

Also, remember that a 75 MHz 603 is not 1/2 as fast as a 150 MHz 603. To put it in a greatly simplified manner, the OS is going to take a certain *number* of clock cycles, not a percentage. (Not exactly, of course; the real answer is somewhere in between.) This difference is most noticeable at the low end (something like the 6220), where the OS is taking nearly all of the computer's power. Of course, older OS versions will take less computing power to run - if you're not already, you may get better performance out of the thing by downgrading to 7.6.1.

>I've also read Chris's replies, and have come to the conclusion that I
>should have a ritualistic burning of my 6220, and put it out of it's
>misery. ;-)
>
>Looks like I'll be updating my DNS server. I'm thinking G3 for a web
>server, and moving the 6400/180 from the web server over to DNS. Any
>opinions on a 6400/180? Or should I just bite the bullet and jump to a G3,
>which seems a little over-kill for DNS.
>Questions, comments, opinions, flames? :-)
>
>So, in conclusion, there is no real generic answer, or even rule of thumb
>to "How much RAM and CPU" does my DNS server need?

Right. Empirical evidence is the best way.

Also, keep in mind (since you have a large customer base rather than a large number of domains) the discussion on the list recently about how most machines use DNS resolvers. The first resolver listed is queried, then allowed to time out, before the machine will query the second resolver listed.

Assuming you have two DNS servers, it would be interesting to see how much traffic actually reaches your secondary server. This would probably reflect the approximate ratio of your customers who put them in in the order you list them, vs. reversing them.
____________________________________________________________________
Chris Buxton cbuxton@menandmice.com
Men & Mice http://www.menandmice.com
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