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Re: [linux-lvm] Q: LVM over RAID, or plain disks? A:"Yes" = best of both worlds?



hansbkk gmail com wrote:
On Tue, Nov 30, 2010 at 1:57 AM, Nataraj <incoming-redhat rjl com> wrote:
Your proposed solution is a bit confusing to understand,

Thanks for taking the time to give me feedback! And sorry I couldn't
make it clearer, but if the diagram is pasted into a monospace screen
it becomes more so - duplicated below.

The key is that I want to RAID1 from a given **LV** within the
TopRAID1 set to regular partitions on physical disks on the TopRAID2
side.

however raid1 works for doing backups in the manner that you describe .
I use it myself and I
have, over time read about others doing so as well.  Be sure to create your
volumes with --bitmap=internal, that way when you swap in a drive, it won't
need to replicate the entire drive, only the part that is changed.

TopRAID1's LVM is likely to be running over a RAID6 set , so I'm not
depending on the TopRAID mirroring for reliability, just using it for
the above volume cloning.
Your raid 1 backups won't mirror any snapshots of your LV's unless you specifically setup mirroring of the snapshots after they exist.
When TopRAID1 is running with the TopRAID2 side marked faulty or
missing, will there be a performance hit or other negative
consequences?
My guess would be no, but the only way to really tell is to run tests. It would depend somewhat on how your drives and controllers are configured, but writes might be faster with only a single drive online, whereas reads may be slower, since you don't have the ability to read from both drives. But since your LV is already raid 6, it's hard to tell what the behavior of raid 1 on top of raid 6 will be. I think you would just have to try it. The syncing of data when you readd a drive into the raid 1 set tends to be very fast (with the internal bitmap).

If so, would it be possible/better for the host In normal operations
to mount the underlying LV directly rather than the degraded top-level
RAID1?

No, you want to have mdadm assemble the raid volume, even if in degraded mode with only one drive and then access the LV on top of the md device. Even if you were able to mount the LV and bypass raid, that would be pointless because you would not update the bitmap and superblock and the integrety of the raid set would be lost.


Nataraj



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