We had a little downtime today between around 12:10 and 13:20. About 5 minutes of this downtime were planned - we simply wanted to reboot the machine. Unfortunately, the machine did not come back up after the reboot, and it took a while to figure out what the problem was.

As it turns out, the machine was unable to mount the huge volume with all our mirrors on it during boot. Manually mounting the disk failed as well because the device just wasn’t there. In lvdisplay the volume was listed as unavailable, and trying to set it available with lvchange failed with the message /usr/sbin/cache_check: execvp failed: No such file or directory Check of pool bigdata/ftpcachedata failed (status:2). Manual repair required!

Recently, we’ve frequently reached the I/O capacity of our RAID array during peak hours, meaning that increasingly often downloads were not limited by network speed, but by how fast our disks could deliver the data.

While we use a hardware RAID6 which does deliver pretty decent read speeds, we’re still using traditional magnetic hard drives, not SSDs. While these arrays easily deliver more than a Gigabyte per second when reading sequentially, this performance drops rapidly the more random the I/O gets, i.e. the more different files are requested at the same time. Of course, due to the nature of our FTP, which provides mirrors for a whole bunch of different projects, we do get a large amount of random I/O.

Since we moved the ftp to the (then) new hardware in October 2013, we had been using XFS as the filesystem on which we store all our mirror trees. All data resided in one single large XFS filesystem of 35 TB. That worked rather well until we updated the operating system on the machine in January.

After the update, the machine behaved extremely unstable - and in crass contrast to its 460 days uptime before the update. Sometimes all file I/O would stop for a few minutes (up to 30), and then suddenly continue as if nothing had happened. Those hangs happened in average twice a day. Sometimes the machine would also lock up completely and need to be reset. During the hangs, the machine would log many of the following error-messages to the kernel log: