Storing UFS snapshots externally?

[Ivan Voras]

Daniel E. Lynn wrote:
> Greetings all,
> 
> First, let me apologize if this has been asked before. If it has then my 
> search skills must be lacking because I couldn't seem to find it in the 
> lists anywhere.
> 
> I'm wondering if it is feasible to store snapshots for UFS on a separate 
> drive, and if so if there is an advisable way of doing it. The basic 
> idea is that I'd like to be able to mitigate any write overhead of using 
> a lot of snapshots by using a separate disk for them entirely. Here's 
> the proposed setup:
> 
> FreeBSD (/) is on ad0
> Homedirs and userdata (/data) is on gm0 (ad2+ad3 mirrored)
> 
> I've been successfully using snapshots for /data, and the overhead on 
> this system doesn't seem too bad (yet) but if I have more than a dozen 
> snapshots, I get the feeling it could get messy. It'd be great if I 
> could store the snapshots for /data on / someplace. 

UFS snapshots don't copy the data into the "snapshot" file - they just 
adjust internal references in the file system. The big file you get when 
you create the snapshot isn't really a file in the traditional sense - 
it consists of file system internal pointers to real data. Simplified, 
when data gets changed on the "real" file system, *then* the old data 
gets a separate copy in the snapshot.

In short, there's no way other than manually copying (dd, tar) the data 
from the snapshot to wherever.

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