Yesterday I talked about how writing things to USB drives made my machine not very responsive , and in a comment Nolan pointed me to LWN's The pernicious USB-stick stall problem . According to LWN's article, the core problem is an excess accumulation of dirty write buffers, and they give some VM system sysctls that you can use to control this.

I was dubious that this was my problem, for two reasons. First, I have a 16 GB machine and I barely use all that memory, so I thought that allowing a process to grab a bit over 3 GB of them for dirty buffers wouldn't make much of a difference. Second, I had actually been running sync frequently (in a shell loop) during the entire process, because I have sometimes had it make a difference in these situations; I figured frequent sync s should limit the amount of dirty buffers accumulating in general. But I figured it couldn't hurt to try, so I used the dirty_background_bytes and dirty_bytes settings to limit this to 256 MB and 512 MB respectively and tested things again.

It turns out that I was wrong. With these sysctls turned down, my machine stayed quite responsive for once, despite me doing various things to the USB flash drive (including things that had had a terrible effect just yesterday). I don't entirely understand why, though, which makes me feel as if I'm doing fragile magic instead of system tuning. I also don't know if setting these down is going to have a performance impact on other things that I do with my machine; intuitively I'd generally expect not, but clearly my intuition is suspect here.

(Per this Bob Plankers article , you can monitor the live state of your system with egrep 'dirty|writeback' /proc/vmstat . This will tell you the number of currently dirty pages and the thresholds (in pages, not bytes). I believe that nr_writeback is the number of pages actively being flushed out at the moment, so you can also monitor that.)

PS: In a system with drives (and filesystems) of vastly different speeds, a global dirty limit or ratio is a crude tool. But it's the best we seem to have on Linux today, as far as I know.

(In theory, modern cgroups support the ability to have per-cgroup dirty_bytes settings, which would let you add extra limits to processes that you knew were going to do IO to slow devices. In practice this is only supported on a few filesystems and isn't exposed (as far as I know) through systemd's cgroups mechanisms .)