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Re: [linux-lvm] Mirror between different SAN fabrics

I'm wondering how well a "stacked" lvm approach would work for this. Could you take LVM and make a two VGs called "Fabric1VG" and "Fabric2VG", where you put all of the "fabric 1 paths" to your PVs into Fabric1VG, and all your "fabric 2 paths" to your PVs into Fabric2VG. Then form volumes in each of those (it would be your responsibility to ensure that you have equal volumes in each fabric group). So you would have Fabric1VG/LVa and Fabric2VG/LVa as "equivilant" devices, going across your different fabric paths. Then you could create a new VG called MirrorVG and put both Fabric1VG/LVa and Fabric2VG/LVa into MirrorVG as PVs. Then I think you should be able to create a new mirrored LV from MirrorVG which would mirror across your two fabrics. This should move all of your management into LVM2, which is cluster aware, but it 1) makes management a bit messy (but it is all quite scriptable), and 2) adds a wierd LVM 2-layer setup.

Anyone know if this 2-layer LVM approach would kill performance (any more than a two layer approach of lvm->mdadm or mdadm->lvm)?

Hmm... Thinking about this further, I've been thinking of this 2-layer approach for striping and mirroring, but it looks like it might be more problematic for your case, where you really want multipath and mirroring. I'm not sure how to ensure that Fabric1VG/LVa and Fabric2VG/LVa are placed on the same PV blocks as the other one. When you create Fabric1VG/LVa it might end up on disks 0,1, and 2, but Fabric2VG/LVa might end up on disks 1, 2, and 3 (but by different paths). Anyone know any way to ensure that block placement is identical? Is the block allocation algorithm predictable and repeatable so that if you have two VGs with equal PVs in each, and you create LVs in the same order in each, they get the same PV mapping? Or is there a built-in randomness? You would likely be assured of the same failures, since a bad block on fabric 1 should also be a bad block on fabric 2, and a failed PV would fail in both LVs... Might work, if the placement routines are predictably defined.

mathias herzog postfinance ch wrote:
At that stage the md stuff is only ever accessed on a single node, and there's no problem.

I will use GFS Filesystem with all cluster nodes up and running at the
same time, sharing their disks.
So my problem with the lvm2 mirroring feature still exists. Think I have
to use the expensive and not easy configurable Continuous Access
Solution to mirror directly on SAN fabric level...


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