On Fri, Jul 27, 2018 at 3:51 AM, Eric Robinson <[email protected]> wrote:
> > 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 It depends I would say it is not unusual to end up with a setup where dbrd is sandwiched between top and bottom lvm due to requirements or convenience. For example in case of master-master with GFS2: iscsi,raid -> lvm -> drbd -> clvm -> gfs2 Apart from the clustered lvm on top of drbd (which is RedHat recommended) you also get the benefit of easily extending the drbd device(s) due to the underlying lvm.
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