So in bug 592422 we started fallocating individual cache files (not _CACHE_00x) up to 1 MB. For files larger than that, we don't want to fallocate the whole size right away (in case they wind up failing to download completely, and/or for possible perf reasons on older filesystems). But we could keep track of the bytes written and fallocate(MIN(remaining, 1 MB)) as we exceed previously fallocated space, so larger files are less fragmented.

new cache code

So does the new cache have this feature?

No, but we write the data in 256kB chunks, so the files should be much less fragmented than in the old cache. Not sure if fallocation would bring a performance gain.