Backup using mirroring

[Todd McDaniel_1]

Okay my DBA emails me asking about fast backups using mirroring... IE, split mirrors and use m1 for backup...

I guess to take a cold backup of the m1 and either keep the m0 running.

I can only guess at this point.

I guess to then resync later... not exactly sure...

Any advice on the trepidation of doing this? Is it a viable option? is it inherently dangerous...

I will post back when I know more if it is needed.

[Pete Randall]

Todd,

Yes, it's absolutely a valid method. It's even taught in HP's "Hands On With LVM and Mirror/UX" course.


Pete

[Todd McDaniel_1]

i think they are going to try and use it like pseudo BCV snapshot ... more to come.

[Todd McDaniel_1]

Pete,

I had that course but it has been a few years...

Would that be with lvsplit or simply lvreduce -m1? How would I go about it?

I am very knowledgeable about LVM, just not in this manner.

[Jeff Schussele · Accepted solution]

Hi Todd,

Yes it can be done this way. But it's much safer to use a 3-way (-m 2) for this because with a 2-way mirror, when you split one away you'll have an SPOF for the time that it's not in the mirror set.

So you would do a
lvreduce -m 1 /dev/vg_name/lv_name /dev/dsk/cXtYdZ
to split it off
And a
lvextend -m 2 /dev/vg_name/lv_name /dev/dsk/cXtYdZ
to bring it back & sync it.

We did this with OpenMail datastore disks all the time. It assures the data is completely inactive for a backup of the data on that disk.

Rgds,
Jeff

[Paula J Frazer-Campbell]

Hi
As said a very viable method, BUT

I would suggest using a tripple mirror on a live important database - spit one away and you still have a mirror safty net.

Basically
1. Split the mirror
2. Mount the mirror split
3. Back it up
4. Join the mirror.


Paula

[James A. Donovan]

Jeff is right, but he got the commands wrong. You must use lvsplit and lvmerge, and NOT lvreduce/lvextend!

[Todd McDaniel_1]

thanks jeff and paula...Great ideas, we use that with a BCV setup on my boxes for my application...

This however, is for an application unlike mine...they dont have BCVs.

heh my company is a big one and requires red tape/capacity planning/Project Creation, et al for disk purchases...long and drawn out..

they only have 2 mirrors m1 and m2... I fear they wont have time or $$ to get additional disks...

also, we are talking about 1TB of data here... across several fielsystems/LVOLS...

As I think about it more... I think they are going to be shooting themselves in the foot by trying to find a shortcut....

[Jeff Schussele]

lvsplit has the added benefit of not having to backup the data raw. It creates a new LV with the name
/dev/vg_name/lv_nameb
it suffixes b to the org lv_name.
You have to fsck the "b" LV before you can mount it for "cooked" access.
You then umount the b LV use lvmerge to rejoin them.
But the same rule applies - A 2-way mirror split any way will leave you exposed for the amount of time it's not in the mirror set.

Rgds,
Jeff

[Jeff Schussele]

Hi Jim,

No it's not wrong - it just depends on the backup method you intend to use.
You can lvreduce/lvextend if you are going to backup it up raw - like using dd or such.
There's more than one way to skin a cat my friend.

Rgds,
Jeff