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Re: using FSevents for backup - is it reliable enough?
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Re: using FSevents for backup - is it reliable enough?


  • Subject: Re: using FSevents for backup - is it reliable enough?
  • From: James Bucanek <email@hidden>
  • Date: Fri, 14 Jan 2011 08:51:23 -0700

Dominic Giampaolo <mailto:email@hidden> wrote (Thursday, January 13, 2011 9:12 PM -0800):

... I can demonstrate one (perhaps pathological) case where Time Machine fails to back up a file.

Alan originally contacted me about this issue, and I can now confirm that the scenario he described is a pretty serious problem.

We are aware that it's a real problem but the question
is how common is this scenario?  To us it seems quite
unusual to rename a directory, do stuff inside it,
rename it back *and* do nothing else that would bump
the mtime on the directory.

I'm beginning to think it's more common than I originally thought. For example, I have one customer who has documented a reproducible FSEvent failure. The missed change event occurs deep inside a virtual machine package directory that contains lots of working and temporary directories.


I agree that it's unlikely that one would rename a directory, change a file inside it, and rename it back without otherwise changing its modification time or anything else. (I'm going to try to document some real-world instance of this today or tomorrow.)

I think the real problem occurs when the significant change is in subdirectory or two removed from the directory that's getting renamed. So even if the renamed directory is touched, there's still no way to determine if one of the subsidiary items has changed without an exhaustive search of its contents.

From a philosophical standpoint, "unusual" is immaterial. FSEvents needs to reliably report all change events. Advertising that my backup solution will incrementally capture "most of your changes" isn't a big selling feature. ;)

I'm actually rather surprised this is happening. When I talked to an Apple
Engineer at the time when Time Machine was introduced (WWDC 2006?), I though they said that the FSEvents were tracked using the nodeID of
the modified directory, not its path. So the path reported in the historic
event stream should be the path of the modified folder as it exists now (i.e.
Users/james/Desktop/Test/a/b/c/d/e/), not its path when it was modified (i.e.
not Users/james/Desktop/Test/x/b/c/d/e/).


I'm not sure how told you that but it's not true.

I don't either. 2006 was a long, long, time ago. It's like BIP (Before iPhone).


It
has been proposed as a possible solution but you can't
just do that since when a file is deleted you want to
know about that but there's not inode number any more.

I'm sure there might be other reasons, but that one shouldn't be a problem. FSEvents report change events for the enclosing folder, not the item that was deleted, so the nodeID of the deleted item is moot. My application will receive the change event for its enclosing directory, and it's my job to detect that one of its enclosed items is now gone.


I can, however, see a more general problem with storing the nodeID for deleted items in the historic log. I don't know how nodeIDs are recycled, but I can see where detecting whether a nodeID has been subsequently reassigned could prove to be problematic.
--
James Bucanek


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  • Follow-Ups:
    • Re: using FSevents for backup - is it reliable enough?
      • From: Alan Snyder <email@hidden>
References: 
 >Re: using FSevents for backup - is it reliable enough? (From: Dominic Giampaolo <email@hidden>)

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