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Replicated Luns by CA

Henry A.C. Jansen
Trusted Contributor

Replicated Luns by CA

Hi Guru's

 

I'am not into Linux so I thought about asking this community. I have a Linux box (RHEL) with several Luns presented from an EVA4400 working all fine. Now we have put a P6300 next to it and are planning to replicate these Luns with the Tempory CA license within CVE 9.4 to the other box. This wil work allright I guess .. but is the linux box capacble of using the replicated Lun on the P6300 once we shut it down - failover the DRG - and boot it up again ?? A little bird whispered that the WWN's of the target are changed so the Lun binding on the Linux box doesn't know this Lun anymore... 

 

Can someone put some light in the darkness ??

3 REPLIES
gstanden
Occasional Advisor

Re: Replicated Luns by CA

Hi sir,

since there are no other answers, here's what I can offer.  If it's a dumb answer, sorry, let's hope it helps somehow.  I'm doing something similar but using DRBD to replicate LUNs across 2 linux boxes.  Linux multipath is what you will need to be able to make use of both or one or the other of the available LUNs across the 2 boxes.  If the boxes are both up, whatever is using the LUNs should be able to get over to either storage replicant using multipath.

Gilbert Standen
gstanden
Occasional Advisor

Re: Replicated Luns by CA

Also, I should have mentioned you will probably need to set up some rules in /etc/udev/rules.d to get the lun wwid to /dev/sd* mappings to be persistent across reboots.
Gilbert Standen
Matti_Kurkela
Honored Contributor

Re: Replicated Luns by CA

I assume you're using a currently-supported version of RHEL (i.e. RHEL 4 or newer). While using multipathed SANs is possible on RHEL 3, it's a pain.

 

The results will depend on how your LUNs are configured: are you using LVM? Custom multipath aliases or default names? Or traditional fdisk partitioning and mounting by device name in /etc/fstab?

 

If you're using LVM on your LUNs, they will keep working without a hitch, as long as the system has some way of seeing the disks. Each time the system boots and each time new disks/LUNs are added, LVM will look at all the disks visible to the system, and search for LVM headers on them. LVM assumes the underlying device names may change, so it does not rely on device names or even WWNs to recognize the disks, only the content. (If LVM is restricted to use only particular devices by custom rules in /etc/lvm/lvm.conf, these may prevent LVM from finding the LUNs after WWN change. It would be best to allow LVM to look at all generically-named multipath devices at least.)

 

If you're using dm-multipath and the WWNs are changed, the multipath subsystem will generate new generically-named multipath devices for the new WWNs. (If you're using the default "friendly names", you'll get a new set of /dev/mapper/mpathNN devices. If you have disabled "friendly names", your multipath device names are essentially WWNs, so it should be obvious that they will change.) If you have defined custom aliases in /etc/multipath.conf, you will obviously have to update the WWNs on them before they will work again, then run "multipath -F; multipath -v2" to flush and re-establish the kernel-level multipath configuration.

 

If you're using traditional fdisk partitions on LUNs, you can use LABEL= or UUID= syntax for specifying the disk devices in /etc/fstab instead of traditional device names. Both the LABEL and the UUID will refer to the content of the disk/LUN, so if a disk device is specified using them in /etc/fstab, the mount command will search all the available disks and mount the correct one, no matter what the name of the device is. (However, it might mount a single-path disk device instead of the multipath device.)

 

If you have raw database LUNs, it depends on how your database engine handles its devices: Oracle ASM also identifies its database LUNs by content, so the WWN change should not be a problem to it... assuming that ASM is allowed to scan all multipath devices, not only a restricted sub-set of them (check the ASM_DISKSTRING setting in ASM configuration).

MK