It's only temporary

re: It's only temporary

Chris — Mon, 19 Apr 2004 19:11:00 GMT

Hmm. In Unix-based systems such as OS X, this is done by creating a file in /tmp, opening the file (usually create and open are combined into one atomic step) and then deleting the file. The file will remain in memory until it is closed.

re: It's only temporary

Larry Osterman — Mon, 19 Apr 2004 20:20:00 GMT

So the *nix filesystems and cache manager treat the /tmp portion of the namespace specially?

Or is because the inode's been unlinked that makes it behave specially? What happens if the system comes under memory pressure and the file metadata needs to be flushed to disk?

re: It's only temporary

byron — Tue, 20 Apr 2004 01:16:00 GMT

what chris is talking about is different.

first, /tmp isn't special.

if you unlink an open file, the file isn't deleted from disk until the file is closed. basically all it does is remove the file from the directory's file, so it only exists in the inode allocation tables.

it doesn't cause the file to live in ram, etc.

basically there's no need to clean up your temp files.

re: It's only temporary

Larry Osterman — Tue, 20 Apr 2004 03:35:00 GMT

Thanks Byron, I appreciate it.

So the unlink does the equivilant of the FILE_FLAG_DELETE_ON_CLOSE, but not the combination of the two (which is what gets you the "never flushed to disk unless necessary")?

re: It's only temporary

byron — Tue, 20 Apr 2004 06:02:00 GMT

yup, that's correct.

re: It's only temporary

Michael Grier — Tue, 20 Apr 2004 23:14:00 GMT

unlink is superior to FILE_FLAG_DELETE_ON_CLOSE because:

1. The name is gone from the directory immediately. With the NT delete model, the deletion doesn't occur until the last handle to the file object is closed.

2. Someone can't go and change the delete-on-close attibute on the handle behind your back (in case you didn't know, if someone else has a handle on your delete-on-close file, they can set it back to not be delete-on-close.

The two of these together make life a living hell for installation programs.

re: It's only temporary

Larry Osterman — Wed, 21 Apr 2004 15:51:00 GMT

As usual, insiteful comment, Mike.

re: It's only temporary

smee — Wed, 21 Apr 2004 17:42:00 GMT

Hm, "inciteful" or "insightful" ;)

Measure Twice, Optimize Once

Larry Osterman's WebLog — Mon, 03 May 2004 22:04:00 GMT