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XZ
What is an XZ file?
The strongest of the common Unix compressors. Standard for source and package downloads.
What XZ is
XZ is a stream format, written and read from front to back. It is used for archiving.
The extension is .xz, and the full name is XZ Archive. Both matter less than what the file can hold, which is what the rest of this page is about.
Where XZ came from
It dates from 2009.
Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.
The specification is public
Anyone can implement it without asking permission or paying a licence, which is why it turns up in so many programs and why files written twenty years ago still open.
Nothing is thrown away
XZ stores its content exactly. Saving it again changes nothing, so it can be opened, edited and re-saved as often as you like without accumulating damage — which is what makes it a working format rather than a delivery one.
It holds exactly one file
XZ compresses a single stream and has no concept of a filename or a folder. That is why it is so often seen paired with TAR, which supplies the structure it lacks.
Where its compression ratio comes from
It works with a window of 64 MB: the amount of already-seen data it can refer back to when it meets a repeat.
That single number explains most of the difference between one archive format and another. A larger window finds more repetition and takes longer and more memory to do it, which is the whole trade.
What it does to protect itself
XZ carries a checksum, so a damaged file is detected rather than silently mis-read.
A checksum will not repair anything. It tells you the file is wrong, which is the difference between finding out now and finding out from whoever you sent it to.
What opens XZ
xz, 7-Zip and Keka read it, and so do most programs of the same kind.
If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.
Opening it in a browser
No browser reads it.
That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.
It is a delivery format
XZ is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.
What goes wrong with it
The recurring complaints: it holds one stream and cannot describe a folder.
None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.
The facts, in one place
| Extension | .xz |
|---|---|
| Media type | application/x-xz |
| First published | 2009 |