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re: [linux-lvm] Sharing Logical Volumes via NFS on Redhat 8.0


Truly sharing LVs requires a FileSystem that understands shared physical drives (often called a Cluster File System of CFS).  Or you must have someway to enforce that only one computer (node) at a time owns the LV (i.e. often used in High Availability Clusters).

For instance, with the OpenGFS CFS you can take a drive put it into a firewire chassis, then connect the firewire to 2 computers.

Now run OpenGFS on both computers and it will allow both computers direct firewire access to the drive.  (It takes a lot of cache coherency programming to make this work.)

Once you have that, you can export the OpenGFS FS via either Samba or NFS.

Unfortunately, OpenGFS does not work with LVM.  (Sistina sells GFS, but I don't know if it talks to LVM either.)

There is discussion that OpenGFS sub-package "pool" should be replaced with Data Mapper. I hope they do that, because the OpenGFS pool driver does not support software RAID, and a disk failure is not survivable unless you have hardware RAID.  If they were using Data Mapper, in theory a shared software RAID array could be setup.

Also, the EVMS project (from IBM) is working on userland admin tools for data mapper that will allow LVs on shared disks to be administered from either node.  i.e. cluster enabled.

The EVMS project is assuming you have the soon to be release Heartbeat 1.0.1 package installed and an operational cluster setup.

I don't know the details, but I_hope_ the EVMS cluster support will include a way to enforce exclusive ownership of the shared LV until a failover occurs.

Greg Freemyer

 >>  (repost):

 >>  Regarding Sharing LVM LV's via NFS on Redhat 8.0...ie. mounting an LV from
 >>  another machine while already having a LV on the same machine. Is it ok or
 >>  not ok? I was talking to a buddy and he doesn't think it's a problem or an
 >>  issue but...

 >>  the current documentation on Sistina's site states that sharing LV's is 
 >>  dangerous unless it's a SCSI or Fibre Storage Device?...and not only that 
 >>  ...adminisitration can only be done on one node? (is it really possible to
 >>  create/resize lv's from one node/machine to another (that has an LV or was
 >>  it meant to mean that it's only possible on cluster aware network storage 
 >>  devices)??? ....

 >>  http://tldp.org/HOWTO/LVM-HOWTO/x1232.html
 >>  under "Dangerous Operations - Sharing Logical LVM Volumes"

 >>  ie.

 >>  >If you need to do any changes to the LVM metadata (regardless of >whether
 >>  >it affects volumes mounted on other nodes) you must go through >the 
 >>  >following sequence. In the steps below ``admin node'' is any >arbirarily 
 >>  >chosen node in the cluster. Admin node                   >Other nodes
 >>  >----------                   -----------
 >>  >                             Close all Logical volumes (umount)
 >>  >                             vgchange -an
 >>  ><make changes, eg lvextend>
 >>  >                             vgscan
 >>  >                             vgchange -ay

 >>  I'm thinking that it might not be a good idea to try to do maintenance on
 >>  a 
 >>  LV that is mounted on another system...but i think it makes a whole heck
 >>  of 
 >>  alot of sense to create/resize/mount an LV on a system before trying to 
 >>  mount it via an NFS share. :)

 >>  Any comments? I personally don't think it would be an issue either (to
 >>  mount 
 >>  an LV on another box/system via an NFS mount) but i'd rather not take the 
 >>  chance of messing up any of the data on those LV's cuz their stuffed :)

 >>  Oh one other thing...if it has been proven that a Redhat 8.0 2.4-24?.x 
 >>  kernel can be patched to LVM 1.0.6+ ....will the data on those LV's be 
 >>  affected in any way or will it all be ok? :)

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