[DRBD-user] Meta device madness!

[Sam Howard]

Hi.

I've Googled a bit and looked through the last few months of archives, but I
didn't really find what I'm looking for ...

I have an existing Xen | LVM + DRBD 0.7 setup with external meta devices,
and I want to migrate the guests to a new Xen server with DRBD 8.2.6.  In
the process, I was thinking about changing the meta device to
"flexible-meta-disk internal", but I need to add a small amount to the LV to
accomdate the metadevice.

This lead me to the very nice formulas in the docs, and I am just too zonked
to make heads or tails out of them.  Am I doing the math right that you need
(a lot) less than 1M of meta storage for 10GB of DRBD backing store?

Also, if I do the flexible method, how much space to you actually reserve on
the device and hide from the OS/filesystem?

One concern that also came up is, if I grow the LV by X MBs, add the
internal meta, what happens if I need to grow the device again?  The new
storage will be appended to the block device, and normally, I can do an
xfs_growfs to "see" and allocate the new space ... what happens to the meta
area?

I have just too many unknowns to be comfortable right now -- educate me,
please!

Lastly, doing Xen live migration, does internal vs external meta have any
pros or cons or is it not even a factor?

Thanks!
Sam
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