[DRBD-user] Lower device already claimed error

Felix Frank ff at mpexnet.de
Thu Jul 12 10:42:15 CEST 2012

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On 07/11/2012 10:27 PM, Digimer wrote:
> The metadata, when "internal", does get written to the end of the
> backing devices. You can mount the raw device, but of course, few if any
> FS will know what that metadata is, so it could well be scrogged.
> Mounting a backing device read-only *should* be safe, but I haven't
> tried it myself.

Not entirely wrong, but not the real problem, either.

A filesystem on a valid drbd backing device will be smaller  than or
equal to ( raw dev size - metadata size ). Mounting this in and of
itself poses no problem, and the metadata will remain untouched.

Of course, now your quicksync bitmap (in the metadata) is out of sync
with the actual data. Whatever gets written while you bypass drbd will
be, well, unknown to drbd. So if you resume drbd operation at a later
point, there will be changed blocks on your disk that drbd won't see a
need to sync to its peer. If no verify is performed, you keep running an
inconsistent resource (until by chance all those sectors are overwritten
again and thus synced after all).

That's my understanding, at least :-)


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