What is the "the dos compatibility flag" in fdisk?

I fired up fdisk and got this warning:

WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (command 'c') and change display units to
         sectors (command 'u').

I didn't even know that my previous Linux installs where DOS-compatible in any way!

What changes?! The man page says nothing about it.

Thanks for answering.

SamK

Posted 2010-07-16T10:11:39.897

Reputation: 508

Answers

Here is your answer. DOS doesn't allow a partition to start (or end) mid-cylinder. It assumes the partition table is corrupt when it sees this, and not only won't boot off of that partition, it won't boot any partition on the disk.

Basically, unless you're dual-booting, turn DOS compatibility off. Even if you are, unless you're running an old version of DOS/Windows, turn it off.

BMDan

Posted 2010-07-16T10:11:39.897

Reputation: 205

So when fdisk warns me because "Partition 1 does not end on cylinder boundary" is it only about dos compatibility?

If I get it well, overlapping cylinders with Linux are OK if sectors are not overlapping? – SamK – 2010-07-16T13:11:58.903

1Actually, Linux will even let you overlap data if you really, really want. It'll probably generate utter gibberish in your FS, and it's certainly not recommended to try it in rw mode, but it's quite possible, and occasionally, albeit rarely, useful (mostly for testing things). – None – 2010-07-16T13:22:37.583

from googling:

http://ubuntuforums.org/archive/index.php/t-1443700.html

Sirex

Posted 2010-07-16T10:11:39.897

Reputation: 10 321

I stumbled upon this thread post too. The post of srs5694 seem quite right. But the guy guesses a lot. Is there any "official" documentation? – SamK – 2010-07-16T11:19:38.987

link-only answers are not good – phuclv – 2018-11-20T01:16:07.323