Posts

“umount is too slow”

A question we see over and over again is

Why is umount so slow? Why does it take so long?

Part of the answer was already given in an earlier blog post; here’s some more explanation. Read more

“read-balancing” with 8.4.1+

DRBD 8.4.1 introduces a new feature: read-balancing, which is configured in the disk section of the configuration file(s). This feature enables DRBD to balance read requests between the Primary/Secondary nodes. Read more

Monitoring: better safe than sorry…

Stumbling upon the Holy time-travellin’ DRBD, batman! blog post there’s only one thing to be said …

Be strict in what you emit, liberal in what you accept[1. Thanks, Larry]

is simply not true when dealing with mission-critical systems.

It’s ok to be alerted on upgrading a machine because the “old, working” RegEx that did the parsing doesn’t match anymore[1. eg. because /proc/drbd got an additional field]; it’s not a problem to get an email when someone adds the 100th DRBD resource and causes the grep to fail; and so on. Read more

Maximum volume size on DRBD

From time to time we get asked things like this:

I want to use a 10TiB volume with DRBD, is that supported”?

The easiest way to answer things like that is to say look for yourself on the public DRBD usage page – the biggest public device size is ~220TiB, so go figure 😉 Read more

Trust, but verify

DRBD tries to ensure data integrity across different computers, and it’s quite good at it.

But, as per the old saying Trust, But Verify[1. attributed either to Lenin or Kennedy] it might be a good idea to periodically test whether the nodes really have identical data, similar to the checks that are[1. or at least can be] done for RAID sets. Read more

DRBD causes too much CPU-load

The TL;DR version: don’t use data-integrity-alg in a production setup. Read more

Increase vm.min_free_kbytes for better OOM resistance

Depending on your setup and your workload (eg. within a virtual machine with little memory and much I/O) you could get into the situation that the kernel has little memory left, so wants to write some dirty pages to disk, but cannot, because for that it would need some memory free! Read more