[DRBD-user] drbd+lvm no bueno

[Lars Ellenberg]

On Thu, Jul 26, 2018 at 05:51:40PM +0000, Eric Robinson wrote:
> > But really, most of the time, you really want LVM *below* DRBD, and NOT
> > above it. Even though it may "appear" to be convenient, it is usually not what
> > you want, for various reasons, one of it being performance.
> 
> Lars,
> 
> I put MySQL databases on the drbd volume. To back them up, I pause
> them and do LVM snapshots (then rsync the snapshots to an archive
> server). How could I do that with LVM below drbd, since what I want is
> a snapshot of the filesystem where MySQL lives?

You just snapshot below DRBD, after "quiescen" the mysql db.

DRBD is transparent, the "garbage" (to the filesystem) of the "trailing
drbd meta data" is of no concern.
You may have to "mount -t ext4" (or xfs or whatever),
if your mount and libblkid decide that this was a "drbd" type
and could not be mounted. They are just trying to help, really.
which is good. but in that case they get it wrong.

> How severely does putting LVM on top of drbd affect performance?  

It's not the "putting LVM on top of drbd" part.
it's what most people think when doing that:
use a huge single DRBD as PV, and put loads of unrelated LVS
inside of that.

Which then all share the single DRBD "activity log" of the single DRBD
volume, which then becomes a bottleneck for IOPS.

-- 
: Lars Ellenberg
: LINBIT | Keeping the Digital World Running
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