GNU C Library version 2.39

GNU C Library version 2.39

Tested on 6.6.6 and 6.7.3 kernels, on a Fedora 39 system. I have a slow USB device as /dev/sda, which is therefore bdi 8:0. To restrict it to 1 second of dirty data, I need to run two commands as root:

1. echo 1 > /sys/class/bdi/8:0/strict_limit
2. echo $((4 * 1024 * 1024)) > /sys/class/bdi/8:0/max_bytes

Per the documentation for bdi limits, the first command tells the kernel that this device's limits must be respected even if the global background dirty limit isn't reached; my system has a global background dirty limit around 8 GiB, so without this, any limit I set below 8 GiB is ignored.

The second sets the actual limit - in this case, 1 second of writes to the device, which is 4 MiB of data. You can see why strict limits matters here, though - without strict limits, the global background dirty limit would push me to 8 GiB of data before even checking the per-bdi limits, unless I had a lot of writes in progress to other devices. And I want relatively large limits for the global limits, since my laptop has a big battery, and when I build a Yocto image, I've often got many readers in parallel that need time on the NVMe drive, along with writes that can be delayed, and I'd prefer the writes to wait so that I'm blocking on CPU, not on I/O.