“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.

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!