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Re: [linux-lvm] LVM2 on-disk layout



The reason the metadata wraps around is to decrease the likelihood of losing all your metadata if a failure happens during an update.

If you always write from the beginning of the area, then any partial write guarantees a complete loss.  If you always start part-way through, then you only lose data when your metadata approaches the full size of the MDA.  Since this is most likely an issue with small arrays (where there are few disks and you may only have 1 or 2 MDAs), this gives vastly greater chances of recovery.

Also, even in the event of a failure, you are likely to get the beginning of the new data + end the of the old data--which is more recoverable (by hand) than the alternatives.

It's all about increasing chances of recoverability in the event of a failure.

On Jan 5, 2009, at 2:22 AM, Tejas Sumant wrote:

I have few questions on lvm2 layout.

1) Why array of MDA (metadata area) is required? Isnt single MDA sufficient?
2) Which MDA is current?
3) Why the metadata inside MDA wraps around, instead of the offset
pointing to the start of metadata?

Can somebody kindly answers this?
--
Tejas Sumant

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Jayson Vantuyl
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