Compressed Disc Formats

PS2 Servers supports compressed disc formats, enabling you to store large game libraries in a fraction of the disk space while streaming them seamlessly to your PlayStation 2.

Server-Side vs Client-Side Decompression

There are two ways compressed images are streamed:

Client-Side Decoding (ZSO): The server transfers compressed LZ4 blocks directly over the network; the PS2's processor decompresses them in RAM. This minimizes network traffic.
Server-Side Decoding (CHD & CSO): The host PC or router decompresses requested sectors in real time and delivers raw optical disc sectors to the console. The console never knows the file is compressed.

1. Format Comparison Matrix

Format	Algorithm	Typical Storage Reduction	Console Decoding	Server Decoding Support
Raw ISO (`.iso`)	None (Uncompressed)	0%	Native	Desktop · Core · Edge
ZSO (`.zso`, `.ziso`)	LZ4 (2048-byte blocks)	20% – 45%	Native in OPL / RiptOPL	Desktop · Core · Edge
CSO (`.cso`, `.ciso`)	Deflate / zlib	25% – 50%	Limited / High IOP load	Desktop · Core · Edge
CHD (`.chd`)	LZMA / FLAC / zlib	35% – 65% (Best)	None	Desktop · Core (via `libchdr`)

2. Optional Compression Dependencies

PS2 Servers Desktop and Core automatically inspect the Python environment for optional acceleration and decompression modules:

libchdr — Required for CHD format decompression. Included in pre-compiled Desktop packages; installable via pip install libchdr in source mode.
lz4 — Accelerates ZSO block parsing and verification.
zstandard — Provides Zstandard dictionary acceleration.

Desktop Dependency Panel

In PS2 Servers Desktop, click Settings → Dependencies to inspect the detected status of all optional compression libraries. If a library is missing, Desktop displays exact installation commands for your OS.

3. Compressing Your Disc Images

Compressing to ZSO (Recommended for Console Compatibility)

Use maxcso to compress raw DVD ISOs into high-speed ZSO images:

maxcso --format=zso --block=2048 game.iso -o DVD/SLUS_201.74.Game.zso

Compressing to CHD (Best Compression Ratio)

Use chdman (from the MAME toolset) to achieve maximum compression for Desktop/Core serving:

# For DVD games:
chdman createdvd -i game.iso -o DVD/SLUS_201.74.Game.chd

# For CD games (from CUE/BIN):
chdman createcd -i game.cue -o CD/SLUS_201.74.Game.chd

Over UDPFS, a CD CHD with raw tracks (CHD v5 MODE1_RAW, MODE2_RAW or AUDIO) is also listed as a read-only .cue + .bin pair beside its .iso view, for clients that read BIN/CUE. The BIN holds 2352-byte sectors without subchannel data. Desktop/Core and Edge builds with CHD enabled do this; a pair is hidden if a real file has the same name.