ISO 9660, Joliet and UDF: what's the difference?

They're file systems for discs: the way files, folders and names are laid out so a computer can read them. ISO 9660 is the oldest and most widely read, Joliet adds longer names with any characters, and UDF is the newer format that handles very large files and longer names. Burn by Zaweba lets you choose ISO 9660 + Joliet + UDF, ISO 9660 + Joliet, or UDF alone, and switches to UDF by itself when a file is too big for the other layouts.

ISO 9660, the old common ground

ISO 9660 dates from the early days of CD-ROM. Its great strength is that almost anything that reads data discs understands it. On its own its naming rules are very limited, which is why it's rarely used by itself today. Instead it's paired with one or both of the others, which add what it lacks while keeping that wide support underneath.

Joliet, for proper Windows names

Joliet is an extension that sits alongside ISO 9660 and stores longer names with Unicode characters, so file names in any language survive. The ISO 9660 + Joliet layout that Windows writes has limits worth knowing. It takes files up to 2 GiB, which Burn measured at exactly 2,147,483,648 bytes. Joliet keeps names up to 108 characters, and longer names get a shortened form ending in ~1, which Burn warns you about before burning.

UDF, for big files and long names

UDF is the format used by DVDs and Blu-rays for video, and it suits modern data discs well. It handles files far larger than 2 GiB and keeps names up to 127 characters. When you add a file bigger than 2 GiB to a data disc, Burn switches to UDF by itself and shows a notice, so a large video or disk image doesn't stop the burn.

Which one to choose in Burn by Zaweba

The combined ISO 9660 + Joliet + UDF option covers the most ground: readers that understand UDF use it, and older ones fall back to ISO 9660 and Joliet. It's a good choice for a disc that might be read on several different machines.

ISO 9660 + Joliet suits discs for older equipment, as long as no file is over 2 GiB and you don't mind shortened forms of very long names. UDF alone suits discs that only modern computers will read, especially with big files.

Whichever you pick, Burn keeps long and Unicode names and warns before anything would be shortened, so you can rename a file or change the layout rather than discover the problem on the finished disc. The data disc page also shows a live capacity bar, and lets you set the disc name, finalise on or off, write speed and per-file verify, or save as an ISO instead of burning.

For backups made with the Backup Assistant, you don't need to think about any of this. Burn plans the discs, splits files that are too large for a disc into numbered pieces and checks every file after burning. The file system choice matters most when you're burning a data disc by hand for someone else to read.