bigalloc

If I were you, I'd use posix_fallocate to de-sparsify (manifest?) all of the blocks. Then you don't have unpleasant surprises waiting for you later.

> My impression was that readahead would be disabled by suitable
> incantations of madvise(). Is that wrong? It didn't benchmark as wrong on
> toy systems, but I would have to check whether we actually re-tested on
> the big machines.

I looked at mm/filemap.c and found this:

> static void do_sync_mmap_readahead(...) {
> ...
> if (VM_RandomReadHint(vma))
> return;
> ...
> }

So I'm guessing you're safe with MADV_RANDOM. But it might be wise to check the source of the kernel you're using in case something is different in that version.