On Apr 26, 2010, at 5:02 AM, Edward Ned Harvey wrote: >> From: Richard Elling [mailto:richard.ell...@gmail.com] >> Sent: Sunday, April 25, 2010 2:12 PM >> >>> E did exist. Inode 12345 existed, but it had a different name at the >> time >> >> OK, I'll believe you. >> >> How about this? >> >> mv a/E/c a/c >> mv a/E a/c >> mv a/c a/E > > The thing that's still confusing you is the idea that directory names or > locations matter. They don't.
Maybe directory consistency doesn't matter for MS-DOS 1.0, but I'm pretty sure that directory consistency is useful in UNIX. > Remember that a directory is just an inode, with text and data inside it, > which stores an association of child names and child inode numbers. Suppose > "somedir" is inode 12345. Then if you "ls somedir/.snapshot/somesnap" then > the system is reading a version of inode 12345 in a time gone by. At that > time, inode 12345 may have been referenced by its parent using the name > "foo" instead of "somedir" but that won't even matter in this case because > we've only instructed the system to read the contents of a past version of > inode 12345. In this case, we haven't told the system to do anything even > slightly related to any parent of that inode. We're not even going to know > what name was associated with inode 12345 at that time. > > At the time of somesnap, inode 12345 had contents which indicate "a.txt" is > inode 1000 and "b.txt" is inode 1050 and so on. So "a.txt" and "b.txt" will > appear in the directory listing, and if you cat a.txt or b.txt, the system > will fetch inode 1000 or 1050 as it appeared at the time of the snapshot. > > Does that help? I completely understand this. No magic here. > There is no actual entity called ".snapshot" It's a magical thing, just > like there is no actual entity called ".zfs" If you "ls somedir" or "ls > somezfsfilesystem" you will see, that the parent inode does not contain any > reference to anything called ".snapshot" or ".zfs" (Unless you turned it > on for some reason.) Yes. And you agree that the relationship to parent directories does not matter, correct? In other words, a tool that looks at either the parent or child snapshot directories is useless. Put another way, you cannot implement something like time machine using directory-level snapshot subdirectories. > However, if you "cd .snapshot" or "cd .zfs" then there's some magic behind > the scenes that's able to handle that differently. I don't know how they do > that. But I do know it's not listed in the inode like any other normal > child subdirectory or file. 'nuff said -- richard ZFS storage and performance consulting at http://www.RichardElling.com ZFS training on deduplication, NexentaStor, and NAS performance Las Vegas, April 29-30, 2010 http://nexenta-vegas.eventbrite.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
