Compressed RAM disks

There was a big discussion last week about whether zram swap should be the default in a future version of Fedora.

This lead me to think about the RAM disk implementation in nbdkit. In nbdkit up to 1.20 it supports giant virtual disks up to 8 exabytes using a sparse array implemented with a 2-level page table. However it’s still a RAM disk and so you can’t actually store more real data in these disks than you have available RAM (plus swap).

But what if we compressed the data? There are some fine, very fast compression libraries around nowadays — I’m using Facebook’s Zstandard — so the overhead of compression can be quite small, and this lets you make limited RAM go further.

So I implemented allocators for nbdkit ≥ 1.22, including:

$ nbdkit memory 1T allocator=zstd

Compression ratios can be really good. I tested this by creating a RAM disk and filling it with a filesystem containing text and source files, and was getting 10:1 compression. (Note that filesystems start with very regular, easily compressible metadata, so you’d expect this ratio to quickly drop if you filled the filesystem up with a lot of files).

The compression overhead is small, although the current nbdkit-memory-plugin isn’t very smart about locking so it has rather poor performance under multi-threaded loads anyway. (A fun little project to fix that for someone who loves pthread and C.)

I also implemented allocator=malloc which is a non-sparse direct-mapped RAM disk. This is simpler and a bit faster, but has rather obvious limitations compared to using the sparse allocator.