I've seen the following trick to create large, empty files somewhat quickly:
dd if=/dev/zero of=test.dat bs=1024 count=1024
However, this still takes some time when creating multi-GB files. I presume this time is dominated by the disk writing all the
\0s from /dev/zero to test.dat.
Is there any way to "instantly" create a file of arbitrary size, without actually initializing its contents to anything?