Re: what is our answer to ZFS?

From: Bill Davidsen
Date: Wed Nov 23 2005 - 10:51:11 EST

Chris Adams wrote:
Once upon a time, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> said:

It was a nice try but there is a giant gotcha most people forget. Its
only safe to make this assumption while you have all of the
files/directories in question open.

Right, at the time the structures were created removable (in any sense) media usually meant 1/2 inch mag tape, not block storage. The inode was pretty well set by SysIII, IIRC.

Tru64 adds a "st_gen" field to struct stat. It is an unsigned int that
is a "generation" counter for a particular inode. To get a collision
while creating and removing files, you'd have to remove and create a
file with the same inode 2^32 times while tar (or whatever) is running.
Here's what stat(2) says:

Two structure members in <sys/stat.h> uniquely identify a file in a file
system: st_ino, the file serial number, and st_dev, the device id for the
directory that contains the file.

[Tru64 UNIX] However, in the rare case when a user application has been
deleting open files, and a file serial number is reused, a third structure
member in <sys/stat.h>, the file generation number, is needed to uniquely
identify a file. This member, st_gen, is used in addition to st_ino and

Shades of VMS! Of course that's not unique, I believe iso9660 (CD) has versioning which is almost never used.

-bill davidsen (davidsen@xxxxxxx)
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me

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