[DRBD-user] question regarding meta-data

Lars Ellenberg Lars.Ellenberg at linbit.com
Wed Oct 6 13:36:00 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.


/ 2004-10-06 12:57:58 +0200
\ Bernd Schubert:
> Hello,
> 
> > Could anyone tell me what the pros and cons of using 'internal'
> > meta-data vs putting the meta-data on its own partition? I have read
> 
> 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.

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.

> > 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
> 
> see obove
> 
> > is whether one is easier to manage. For instance, if the primary crashes
> 
> Easier to manage are certainly internal metadata, except you are upgrading 
> from a version <0.6.x
> 
> > and needs a rebuild, the documentation notes that you should 'remove any
> > meta-data' before you attempt to sync it back from the acting primary.
> > Is this option available with 'internal' meta-data?
> 
> Hmm, about some parts of documentation I sometimes don't care ;) The metadata 
> make sure that only those parts are synchronized that differ, so I wouldn't 
> delete it (with a very few exceptions where I don't trust the metadata 
> anymore, e.g. when both nodes have been primary for some reasons).

1. step-by-step advice given for drbd 0.6  [ some software some version]
   do not always apply to drbd 0.7         [ same software other version]

2. that advice was given to people who already screwed up,
   or who believed to have done so.

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.


	Lars Ellenberg

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