Harry Putnam wrote:
This may be a bit poorly thought through but in this case I don't
really know enough to really think it through.
My back ground is linux... there I used a tool called rsnapshot which
used rsync and some hardlink magic to create versioned backups. But
take very little space.
By versioned I don't mean as in version control but just copies of
files as they change.
It worked by running what they called an hourly backup then rotating
that out to daily weekly etc. even the hourly term only really meant
a specific kind of run... not necessarily actually done hourly.
But the punch line was you always had a full backup under the
directory created by hourly
All the backups used hard links to just create a name, no duplicate
data, in the rotated directory unless the file had changed. That's
where the space saving came in.
I'm trying to see now how to do something similar with snapshots.
However I don't really understand how Copy On Write works, even though
I read some web pages about it.
So I wanted to hear some examples of how zfs users would handle
something like that.
The end goal being that user can go into these snapshots or whatever
else it may require and retrieve the same file for a day ago or a week
ago, or month etc if desired.
So there is a running copy of any changes going back in time. And all
done in as little disk space as possible.
Recent Nevada and OpenSolaris do this out of the box, and the Gnome
nautilus filemanager is integrated with this, providing a timeslider
facility to slide back in time and see what was in a directory in the past.
http://wikis.sun.com/display/OpenSolarisInfo/How+to+Manage+the+Automatic+ZFS+Snapshot+Service
Copy on Write means that only blocks that have been written since a
snapshot occupy extra space. That could achieve better savings than
symlinks. For example, if you add something onto the end of a large
logfile, only the changed blocks at the end will be in separate physical
disk blocks. All the rest of the blocks in the file which weren't
written to since the snapshot will be shared. (This requires that the
app appends to the file, and doesn't rewrite the whole file just to add
something on the end.)
--
Andrew
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