Re: udev questions

[Paul Thomas <pthomas8589@gmail.com>]

On Thu, Nov 12, 2009 at 3:31 PM, Timo Juhani Lindfors
<timo.lindfors@iki.fi> wrote:
> Paul Thomas <pthomas8589@gmail.com> writes:
>> I'm using emdebian grip lenny. Currently mounting the / over nfs. I've
>
> I'm using normal lenny so what I say here may or may not apply to
> emdebian.
>
>> I don't see any /dev or udev entry in /etc/fstab. How does the system
>> know where and how to create "udev on /dev type tmpfs (rw,mode=0755)"?
>
> /etc/init.d/udev mounts it:
>
>  # mount a tmpfs over /dev, if somebody did not already do it
>  ...
>  if ! mount -n -o size=$tmpfs_size,mode=0755 -t tmpfs tmpfs /dev; then
>  mkdir -p $udev_root/.udev/db/
>
>> Where does /dev/.udev/db come from? It's not on the nfs shared
>
> My guess is that udev generates it. The udev_device_update_db()
> function in libudev-device-private.c does things like
>
> if (udev_device_get_devlink_priority(udev_device) != 0)
>        fprintf(f, "L:%u\n", udev_device_get_devlink_priority(udev_device));
> if (udev_device_get_event_timeout(udev_device) >= 0)
>        fprintf(f, "T:%u\n", udev_device_get_event_timeout(udev_device));
> if (udev_device_get_num_fake_partitions(udev_device) != 0)
>        fprintf(f, "A:%u\n", udev_device_get_num_fake_partitions(udev_device));
>
> and the lines in the files in /dev/.udev/db have exactly this format.
>

OK, .udev/db is just udev tracking what's in sysfs. So if it's a
device in /sys then in takes up space in /dev on the udev tmpfs (i.e.
ram which is limited). In the kernel I configured
CONFIG_LEGACY_PTY_COUNT from 256 to 16 and this reduced .udev/db from
~2MB to ~500kB which was enough that /dev didn't run out of inodes
anymore. Also I guess newer udev versions use a smaller db, so maybe
that's already in unstable and it will come down the pipe at some
point.

thanks,
Paul