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drbd-user: Re: [DRBD-user] Detection of Logical Volume on nested

Re: [DRBD-user] Detection of Logical Volume on nested LVM

From: Maros Timko <timkom_at_nospam>
Date: Sat Mar 06 2010 - 17:01:19 GMT
To: drbd-user@lists.linbit.com

>
> Message: 1
> Date: Thu, 4 Mar 2010 17:48:24 +0100
> From: Olivier LAMBERT <lambert.olivier@gmail.com>
> Subject: [DRBD-user] Detection of Logical Volume on nested LVM
> replication
> To: drbd-user@lists.linbit.com
> Message-ID:
> <f769216c1003040848m4b1ffaaatc15dc952781ce946@mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Hello,
>
> I'll try to be clear, but it's really hard to explain..
> In few words : when I create a Logical Volume on a host attached
> (iSCSI) to one DRBD server, it doesn't appear on the second host,
> attached to the other DRBD. But, on existing LV, it works like a charm
> (all data's are replicated). And more, if I create a LV on "one side",
> and fill it with data, I saw the replication link with heavy traffic,
> but always no LV.
> Now, to understand, here is how it happened with this infrastructure.
>
> 1) On both DRBD hosts : Debian GNU/Linux, Dual Primary setup, version:
> 8.3.7 (api:88/proto:86-91). I choose to configure a resource on a
> Logical volume of 500Gb (named /dev/vg0/xen). To be clear, two hosts
> are "DRBD1" & "DRBD2". a cat /proc/drbd gave this :
> 0: cs:Connected ro:Primary/Primary ds:UpToDate/UpToDate C r----
> ns:153716 nr:408752 dw:562408 dr:355156 al:179 bm:109 lo:0 pe:0
> ua:0 ap:0 ep:1 wo:n oos:0
>
>
> 2) This resource is exported with iSCSI. For the example, let's say
> that CLIENT1 is connected to DRBD1, and CLIENT2 is connected to DRBD2.
> It's simplifed because in real, it's multiple Xen Dom0's clients with
> multipath (but we don't care of that here).
>
> 3) CLIENT1 see the device as a block device (so far so good, iSCSI
> works). I choose to use LVM on this block device. I create for example
> : /dev/vg_xen/mydisk on CLIENT1. I mount it, I put some stuff in there
> : I saw the replication like working : some traffic. DRBD2 and 1 says
> it's all OK. If I lvscan on CLIENT1, I can see my brand new volume.
>
> 4) CLIENT2, (so, connected on iSCSI with DRBD2), see the block device,
> the volume group, but NOT the Logical Volume. If I disconnect DRBD2's
> resource, and reconnect it, and reconnect iSCSI of CLIENT2, wow, I saw
> the LV !
>
> And more : if the LV exists on both side (after disconnect/reconnect
> the resource), data's are correctly replicated (obviously, I do NOT
> mount LV on both side, I'm aware of that !). But, if I fill on one
> side (e.g CLIENT1), dismount it, then mount it on CLIENT2, data are
> here, without any problem.
>
> So, my "theory", is that LVM operations (lvcreate or lvremove) on a
> volume group, which is on top of iSCSI and LVM replicated device by
> DRBD, are NOT replicated, UNTIL disconnect/reconnect the ressource. I
> don't know why, and that's why I ask here to understand what I miss.

Right. Discussed many times here and on Heartbeat/Pacemaker list. Try
to search with cLVM keyword.

> Additional Informations: If my clients are connected to the SAME DRBD
> (let's tell DRBD1), if CLIENT1 creates a LV, CLIENT2 is immediatly
> aware of that (just inactive, not a problem, vgchange and it works).
> So the "problem" is during the replication.
>
> Thanks for your help.
>
>
> Regards,
>
>
> Olivier
> XO Project
> http://xen-orchestra.com
>
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