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Re: [linux-lvm] snapshot questions



On Friday 02 November 2001 13:59, Kenny Gorman wrote:
> I will try to quicky summarize what _I_ do:
>
> - Yes snapshots work.
> - No, it's not a 1 to 1 disk usage ratio between the vg and the snapshot
> vol.  It's much less.  I dont know how to test for fullness using
> Linux/LVM yet.
> - I use Oracle, your DB may be different.
> 1) put the tablespaces in hot backup mode
> 2) take a snapshot (mount the snapshot vol) of the volume/s that the
> tablespace is on
> 3) take the db out of hot backup mode.

I use MySQL in a mode that uses files in the filesystem.  Does your
Oracle installation use raw disk or files?  If it is raw disk, then
I probably need to do something that will flush disk changes somehow.
I am hoping that the VFS hooks will make the filesystem flush its 
buffers to disk so that I don't get a snapshot full of garbage.  I
suppose that Oracle will flush all buffers to disk (thus making a
coherent disk image) when you put it into hot backup mode.

When I tell MySQL to flush its tables, it simply writes out everything it
can, closes and reopens the files.  Thus it is up to the filesystem/OS to
get the data to the disk in a reasonable fashion.

Interestingly, this may imply (and this seems to be in line with some of
the responses I read) that journaling filesystems may actually be more
of a problem than non-journaling ones.  Weird, but seems to make sense.

> - I also do some checkpointing, etc in my script.

We keep live remote replicas, but sometimes the replica needs to be
rebuild.  In that case, it is very important to have a means of getting
a completely coherent database snapshot.  Right now, this process is very
carefully done with lots of highly database specific code.  Ick.  It is a
pain to maintain and has a lot of edge cases that we find from time to
time :-)  We'd like something a little simpler and more complete.

> - the tape drive can then get the backup from the snapshot.

In our case, we spool it off remotely, but I get the idea.

> - You would restore the image from tape, to a new or the orig. volume
> when you recover.
> - no noticable effect on end users, it's a 24/7 thing.  Restores work
> fine.

So you have this in production?

> It's really not all that scary. Sorry for not going into more technical
> detail.

No problem.  I am still figuring out how to deal with LVM.  It looks like
the solution to all our problems, but I want to make sure before we spend 
more time on it.  Our existing solution works, but it costly to maintain.

Best,
Kyle

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