[DRBD-user] question regarding meta-data

campbellm at cia.com.au campbellm at cia.com.au
Thu Oct 7 03:01:00 CEST 2004

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


> > 
> > 
> > I guess one of the reasons to have this option is for users upgrading from a 
> > previous drbd version (<0.7.0) to >= 0.7.0. 
> > E.g. people having a drbd-0.6 device will have a problem upgrading to 0.7.x 
> > since drbd now needs 128MB of metadata on the underlying device and so one 
> > would be forced to shrink the filesystem. As you might guess fs shrinking is 
> > always pretty dangerous. So setting up the meta data anywhere else might be 
> > the only solution in those cases.
> even though we do our best to reduce meta-data updates,
> which are additional small synchronouse writes generated within the
> applications (file systems) write-out path, they do occur.
> if you have a large device with internal meta data, this involves
> seeking from the active set somewhere to the very end of the device,
> and back again.

In our case, the disk itself is only 73GB, and the DRBD partition is 
only 40GB, and the the machine is mainly acting as a web server, so 
large numbers of writes are not likely to be an issue.

> if you put it on a separate (physical!) device, this is not an issue.
> also, if you put the meta data on a differen (this time it may even be
> logical) device, then resizing the drbd device later will be much less
> cumbersome, and less risky, too. unfortunately they are also easier to
> mix up, if you reconfigure frequently. drbdadm just believes its config
> file. there is not too much paranoia in place to avoid the admin to
> shoot himself.

Yes - I can see how added complexity could also add to the potential for
an admin stuff up

In the case of migrating an existing DRBD device to a new disk (e.g., if
you needed to move to a larger disk), could you use a utility like 'tar'
to back the DRBD device up, configure the new DRBD device, and then tar
the data back? Will this cause problems with meta-data from the old
device? I am guessing that 'tar' and 'cp' (what about 'dump') won't see it as
it not part of the filesystem as such...

Out of curiosity, how is the meta-data itself stored when it is

> > > all the documentation (as far as I can see), but am not really sure
> > > which is more suitable. It mentions in the documentation that if you
> > > use meta-data internal, then you should 'shrink an existing filesystem
> > > first'. I'm not sure what this means. The main reason behind my question
> > 
> 3. if you really screw up, and you feel the need to fiddle with the drbd
>    meta data, there is a write_gc.pl script ...
>    but you should understand the details of the drbd GCs, and double
>    check that script, before you use it.

I can't see myself wanting to do that anytime soon.

Thanks for your time in filling in some of the blanks here!



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