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Veger
Joined: 14 Nov 2009
Posts: 4
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 Posted: Sat Nov 14, 2009 12:58 pm Post subject: VV2.5 and Windows7 junctions and hardlinks |
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I find hardlinks and especially junctions very useful for organizing my data and documents.
What happens in VV2.5: are folders/files followed across junction/hardlinks?
My opinion: do not follow them!
I hope that you agree with me,
greetings
Peter J. Veger, Best Netherlands |
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TGRMN Software
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Joined: 10 Jan 2005
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pb
Joined: 04 Sep 2007
Posts: 32
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| Posted: Tue Mar 30, 2010 7:55 am Post subject: symlinks and hard links |
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The other side of this coin is that if you want to RESTORE your symlinks and hardlinks, VV has not backed them up. My opinion is that VV should give you the option of saying what you want done with these. Note that because windows (at least up to xp) only supports absolute symlinks, it is tricky to back up and restore these, because the restored symlink can refer to a different volume if it was restored to a different volume. The same is true of the backup. You can find some info online about how tar deals with these issues.
Ignoring hardlinks also raises other questions. I'm not even sure whether in windows you can tell the difference between the first link and additional links. But even if you can tell this from a program, I usually do not recall which was the original and which was the second or further link. Then, when I restore, stuff will be missing.
Even if your only goal is synchronization, you can't synchronize two directories if you don't have the same symlinks and hardlinks in them.
Do you have any plans to fix up these problems? Does using the shadow copy feature allow this?
--peter |
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TGRMN Software
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| Posted: Tue Mar 30, 2010 12:49 pm Post subject: |
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I think the main problem is that copying a symlink / hardlink only (that is the pointer to the data, without the data pointed to) would also not guarantee a full backup. Also in Windows 7 these links between folders can be recursive, e.g. they program can go into a loop trying to following all the links.
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pb
Joined: 04 Sep 2007
Posts: 32
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| Posted: Tue Mar 30, 2010 6:28 pm Post subject: symlinks and hard links |
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| TGRMN Software wrote: | | I think the main problem is that copying a symlink / hardlink only (that is the pointer to the data, without the data pointed to) would also not guarantee a full backup. Also in Windows 7 these links between folders can be recursive, e.g. they program can go into a loop trying to following all the links. |
Well, that's all true, but tar has dealt with these problems for the past 2 or more decades. (However, unix has the advantage of using relative symlinks. I think tar might refuse to back up absolute symlinks, but I'm not sure.)
Certainly detecting loops can take arbitrarily large amounts of memory and time (in principle), but it can be done.
But, I'm not sure you understand my point. I am not arguing that you should back up what a symlink points to (although backupexec, for example, does give you that choice, as does ntbackup, or more exactly ntbackup gives you the choice when you restore). What I would like to be able to back up is the symlink itself. The only problem then is that the symlink will continue to point to wherever it used to point to before, which might not be what the user expects. Probably the right way to approach that is to determine whether the symlink is pointing to the same filesystem as what is being backed up; if so, it should be converted to point to the target filesystem, and if not, it should be left alone. (And the same on a restore.) There are other options also, for example to maintain a database of these things, like your tdb.
But, sometimes that suggestion might not be what a user wants, for example if he is copying to a different place in the same file system. Then one needs to look at whether the original symlink pointed to a place above or below it in the file tree. If below, then there is a good chance that the user would like it to point to the analogous place in the target. However, there is no way to fully analyze this in the program, so you would need to offer the user options as to how to handle this.
In my case, I use both symlinks and hardlinks liberally to avoid duplicating data. Currently with VV that causes a severe limit on my ability to restore my file systems if they contain symlinks or hardlinks. Hardlinks are even more thorny if they are not backed up and restored, because it is very difficult to detect at the user level what files are multiply linked.
--peter |
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pb
Joined: 04 Sep 2007
Posts: 32
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| Posted: Tue Mar 30, 2010 6:36 pm Post subject: symlinks and hard links |
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| TGRMN Software wrote: | | I think the main problem is that copying a symlink / hardlink only (that is the pointer to the data, without the data pointed to) would also not guarantee a full backup. Also in Windows 7 these links between folders can be recursive, e.g. they program can go into a loop trying to following all the links. |
Oh, I should add one more point. Backing up the symlink itself IS a full backup in the sense that all the information that is actually on the disk is being backed up. If I have a shortcut on my desktop to a url, you wouldn't claim that if I don't copy what is on the web page pointed to by the url, then I don't have a full backup of my desktop, would you? Urls can have loops, too. The situation with symlinks is exactly the same; urls and symlinks are the same thing -- simply pointers to another place where the data is.
On the other hand if the shortcut happens to point to a url that ultimately resolves to an html file that just so happens to also be on my desktop, then the target of the url will be backed up, and none of that should be terribly bizarre to anyone. The only difference is that the file system tries to hide the fact that the symlink is really just a pointer. |
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pb
Joined: 04 Sep 2007
Posts: 32
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| Posted: Tue Mar 30, 2010 6:41 pm Post subject: Re: symlinks and hard links |
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| pb wrote: | | TGRMN Software wrote: | | I think the main problem is that copying a symlink / hardlink only (that is the pointer to the data, without the data pointed to) would also not guarantee a full backup. Also in Windows 7 these links between folders can be recursive, e.g. they program can go into a loop trying to following all the links. |
Oh, I should add one more point. Backing up the symlink itself IS a full backup in the sense that all the information that is actually on the disk is being backed up. If I have a shortcut on my desktop to a url, you wouldn't claim that if I don't copy what is on the web page pointed to by the url, then I don't have a full backup of my desktop, would you? Urls can have loops, too. The situation with symlinks is exactly the same; urls and symlinks are the same thing -- simply pointers to another place where the data is.
On the other hand if the shortcut happens to point to a url that ultimately resolves to an html file that just so happens to also be on my desktop, then the target of the url will be backed up, and none of that should be terribly bizarre to anyone. The only difference is that the file system tries to hide the fact that the symlink is really just a pointer. |
And, I should carry this argument one step further. Programs like wget, which retrieve whole chunks of the web to store locally, typically fix up the urls in the files they are retrieving to point to the local file they have retrieved rather than continue to point to the place on the web where they came from. In addition, such programs control being trapped in loops by using a maximum recursion depth. This is all also exactly the same as the situation for backing up directories with symlinks. |
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