probably asked this before - file descriptors?
You effectively use 0 filedescriptors on the physical host. Well, you are using one for each unique filesystem you setup.. or sompn like that.
Ultimately, any fd limit you hit is something you can correct yourself.
perhaps someone else more clued will chime in here.
I've observed Linodes consuming an average of 20-30 file descriptors on the host. There is a limit placed on each Linode on the host itself, but it is an insanely high value that's only there as a break for run-away Linodes (or other evil-doers).
Inside your Linode, you are free to use as many file descriptors as your UML kernel can handle (or /proc/sys/fs/file-max), which has a compiled in max of 1048576 open fd.