Aaron has checked in a change to the CHD format featuring his further work on the format, and bumping the version to v5.
This includes a refined / cleaned up version of the FLAC support I was working on (which is now official) as well as a number of other improvements (LZMA support based on the 7-zip code is also in there)
I haven’t tested / used this code yet, so I can’t say how compressions compare (if data is being optimally ordered for FLAC etc.) but you should see substantial gains, even over the previous versions I posted due to the use of LZMA which in tests on regular romsets usually yields around a 20% improvement in compression. The LZMA improvements mean there will also be benefits to non-CD CHDs. LD ones are also likely to shrink very slightly as FLAC is being used for the audio part of those now (don’t expect big savings tho, they’ve always been video-heavy and lossless video compression is always going to give big files)
The created CD CHDs are SHA1-compatible with the previous ones, so none of the software lists need updating thankfully.
Just a heads up, because unless there are any bugs this will be the expected version from the next release, as opposed to the previous FLAC trial runs.
In case you haven’t been following any of this here’s a summary:
Old V4 (and below) CHD format used ‘zip’ compression internally.
New V5 format supports ‘zip’ , ‘lzma’ (7-zip) and ‘flac’
FLAC seems to give about 40% better compression than ‘zip’ for audio data (CD AUDIO tracks)
LZMA seems to give about 20% better compression than ‘zip’ on regular data.
Of course some data just plain doesn’t compress well, so you’re not going to see those saving everywhere but you’re likely to see on average a 20-40% reduction in CHD size for all non Laserdisc CHDs.
This naturally makes CHD better for storing large amounts of data, which was it’s original purpose.