Wandering Thoughts archives

2014-08-31

We don't believe in DHCP for (our) servers

I understand that in some places it's popular for servers to get their IP addresses on boot through DHCP (presumably or usually static IP addresses). I understand the appeal of this for places with large fleets of servers that are frequently being deployed or redeployed; to put it one way I imagine that it involves touching machines less. However it is not something that we believe in or do in our core network, for at least two reasons.

First and lesser, it would add an extra step when setting up a machine or doing things like changing its network configuration (for example to move it to 10G-T). Not only would you have to rack it and connect it up but you'd also have to find out and note down the Ethernet address and then enter it into the DHCP server. Perhaps someday we will all have smartphones and all servers that we buy will come with machine readable QR codes of their MACs, but today neither is at all close to being true (and never mind the MACs of ports on expansion cards).

(By the way, pretty much every vendor who prints MACs on their cases uses way too small a font size.)

Second and greater, it would add an extra dependency to the server boot process. In fact it would add several extra dependencies; we'd be dependent not just on the DHCP server being up and having good data, but also on the network path between the booting server and the DHCP server (here I'm thinking of switches, cables, and so on). The DHCP server would become a central point of near total failure, and we don't like having those unless we're forced into it.

(Sure, we could have two or more DHCP servers. But then we'd have to make very sure their data stayed synchronized and we'd be devoting two servers to something that we don't see much or any advantage from. And two servers with synchronized data doesn't protect against screwups in the data itself. The DHCP server data is a real single point of failure where a single mistake has great potential for serious damage.)

A smaller side issue is that we label our physical servers with what host they are, so assigning IPs (and thus hostnames) through DHCP creates new and exciting ways for a machine's label to not match the actual reality. We also have their power feeds labeled in the network interfaces of our smart PDUs, which would create similar possibilities for exciting mismatches.

sysadmin/NoDHCPForServers written at 21:40:13; Add Comment

The downside of expanding your storage through bigger disks

As I mentioned recently, one of the simplest ways of expanding your storage space is simply to replace your current disks with bigger disks and then tell your RAID system, file system, or volume manager to grow into the new space. Assuming that you have some form of redundancy so you can do this on the fly, it's usually the simplest and easiest approach. But it has some potential downsides.

The simplest way to put the downsides is that this capacity expansion is generally blind and not so much inflexible as static. Your storage systems (and thus your groups) get new space in proportion to however much space (or how many disks) they're currently using, and that's it. Unless you already have shared storage, you can't reassign this extra new space from one entity to another because (for example) one group with a lot of space doesn't need more but another group with only a little space used now is expanding a lot.

This is of course perfectly fine if all of your different groups or filesystems or whatever are all going to use the extra space that you've given them, or if you only have one storage system anyways (so all space flowing to it is fine). But in other situations this rigidity in assigning new space may cause you heartburn and make you wish to reshape the storage to a lesser or greater amount.

(One assumption I'm making is that you're going to do basically uniform disk replacement and thus uniform expansion; you're not going to replace only some disks or use different sizes of replacement disks. I make that assumption because mixed disks are as much madness as any other mixed hardware situation.)

sysadmin/BiggerDiskExpansionIssue written at 00:35:08; Add Comment


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