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Re: [Linux-cluster] Cluster with shared storage on low budget

On 02/15/2011 06:19 PM, yvette hirth wrote:
Thomas Sjolshagen wrote:

Just so you realize; If you intend to use clvm (i.e. lvme in a cluster
where you expect to be able to write to the volume from more than one
node at/around the same time w/o a full-on failover), you will _not_
have snapshot support. And no, this isn't "not supported" as in
"nobody to call if you encounter a problem", it's "not supported" as
in "the tools will not let you create a snapshot of the LV".

i've been listening to this discussion with much interest, as we would
like to improve the currency of our backup files.

right now we have an ensemble of GFS2 LV's ("pri") as our primary data
store, and a "matching ensemble" of XFS LV's ("bak") as our backup data
store. an hourly cron job rsync's all LV's in the ensemble from pri =>
bak. it's incredibly reliable, but this reduces our mean backup currency
by 1/2 hour. one upside is that i've got snapshots that are only 1/2
hour old, and are daily backed up to tape.

the conversation seems to indicate that we can change the bak LV's from
XFS to GFS2 and have drbd auto-sync the pri LV changes made to the bak
LV's - yes? this would reduce our backup currency from a mean of 1/2
hour to theoretically, "atomic" (more likely "mere seconds"). i assUme
we have to change from XFS to GFS2, as drbd doesn't appear to do file
system conversions...

if our assumptions are correct, are there any guides / manuals / doc on
how to do this? it's most tempting to try, since if it doesn't work, the
hourly cron rsync's could be simply reinstated.

I'm not sure you realize what this would require. DRBD is a block device. You would have to start with a new partition/disk, "format" it for DRBD (creates DRBD metadata on the block device), then create GFS on top of it and put your files in. It's a backup+restore job to migrate to and from it.

If you were to do this, your backup node would have to be a part of your DRBD cluster (all nodes need to share the DRBD device, unless you plan to only use it on the SAN that all the nodes connect to the volume from). You would then drop the backup node out of the cluster completely and make sure it cannot reconnect (this is vitally important), mount the GFS FS from DRBD ro with lock_nolock, and then back that up. Unless you are happy with just a block level mirror, which won't help you if data is accidentally deleted. DRBD is network RAID1, and RAID (of any level) is not a replacement for backups - but I'm sure you know that.


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