How to change relatime access time update interval from 24 hours

Ingo Molnar proposed a patch, but this patch wasn't accepted into the kernel tree. Linus Torvalds made relatime the default setting in 2.6.30, unconditionally, and this is still true in 3.0. If you want relatime to default off in the kernel, you need to apply Ingo Molnar's patch in your copy of the source.

Tracking freed blocks is unavoidable in any decent file system and ZFS is no exception. There is however a simple way under ZFS to have a nearly instantaneous directory deletion by "deferring" the underlying cleanup. It is technically very similar to Gilles' suggestion but is inherently reliable without requiring extra code.

If you create a snapshot of your file system before removing the directory, the directory removal will be very fast because nothing will need to be explored/freed under it, all being still referenced by the snapshot. You can then destroy the snapshot in the background so the space will be gradually recovered.

d=yourPoolName/BackupRootDir/hostNameYourPc/somesubdir
zfs snapshot ${d}@quickdelete && { 
    rm -rf /${d}/certainFolder
    zfs destroy ${d}@quickdelete & 
}