[DRBD-user] drbd+lvm no bueno

[Eric Robinson]

> On Thu, Jul 26, 2018 at 04:32:17PM +0200, Veit Wahlich wrote:
> > Hi Eric,
> >
> > Am Donnerstag, den 26.07.2018, 13:56 +0000 schrieb Eric Robinson:
> > > Would there really be a PV signature on the backing device? I didn't
> > > turn md4 into a PV (did not run pvcreate /dev/md4), but I did turn
> > > the drbd disk into one (pvcreate /dev/drbd1).
> 
> Yes (please view in fixed with font):
> 
> | PV signature | VG extent pool |
> |.... drbd1 .... .... .... .... | drbd metadata | .... md4 .... ....
> |.... .... .... .... .... ...|md metadata|
> |component|drives|.....|.....|...of...|md4......|.....|.....|
> 
> > both DRBD and mdraid put their metadata at the end of the block
> > device, thus depending on LVM configuration, both mdraid backing
> > devices as well as DRBD minors bcking VM disks with direct-on-disk PVs
> > might be detected as PVs.
> >
> > It is very advisable to set lvm.conf's global_filter to allow only the
> > desired devices as PVs by matching a strict regexp, and to ignore all
> > other devices, e.g.:
> >
> >  global_filter = [ "a|^/dev/md.*$|", "r/.*/" ]
> >
> > or even more strict:
> >
> >  global_filter = [ "a|^/dev/md4$|", "r/.*/" ]
> 
> Uhm, no.
> Not if he want DRBD to be his PV...
> then he needs to exclude (reject) the backend, and only include (accept) the
> DRBD.
> 
> But yes, I very much recommend to put an explicit white list of the to-be-
> used PVs into the global filter, and reject anything else.
> 
> Note that these are (by default unanchored) regexes, NOT glob patterns.
> (Above examples get that one right, though r/./ would be enough...
> but I have seen people get it wrong too many times, so I thought I'd mention
> it here again)
> 
> > After editing the configuration, you might want to regenerate your
> > distro's initrd/initramfs to reflect the changes directly at startup.
> 
> Yes, don't forget that step ^^^ that one is important as well.
> 
> 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?

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

> 
> Cheers,
> 
> --
> : Lars Ellenberg
> : LINBIT | Keeping the Digital World Running
> : DRBD -- Heartbeat -- Corosync -- Pacemaker
> 
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