[DRBD-user] Documentation tweak for external meta-data

Bill Broadley bill at broadley.org
Fri Jul 2 09:32:59 CEST 2010

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


It took some digging before I realized that the formula for external meta-disk
storage is 128MB * DRBD devices.

I'd suggest changing the man page from:
  You can use a single block device to store meta-data of multiple
  DRBD devices. E.g. use meta-disk /dev/sde6[0]; and meta-disk
  /dev/sde6[1]; for two different resources. In this case the
  meta-disk would need to be at least 256 MB in size.

To something like:
  You can use a single block device to store meta-data of multiple
  DRBD devices. E.g. use meta-disk /dev/sde6[0]; and meta-disk
  /dev/sde6[1]; for two different resources.  The storage required
  is 128MB * number of DRBD devices.

Then where I looked most closely was:
 http://www.drbd.org/users-guide/ch-internals.html

Maybe add a section saying:

18.3 Calculating external shared meta-disk size = 128MB * DRBD devices.

Googling around the most authoritative source I found for this was:
  http://lists.linbit.com/pipermail/drbd-user/2009-July/012443.html

Seems like other people had the same problem:
  http://lists.linbit.com/pipermail/drbd-user/2009-July/012444.html

My approach before I knew about the restrictions on the shared meta data (and
the normal formula not appling was:

drbdnode:
1TB disks      A   B   | C    D
partition 1    raid-1  | raid-1
partition 2    LVM LVM | LVM LVM

So I was hoping the RAID-1 on disk a and b could hold the metadata for any
DRBD devices on partition 2 of disks C and D.

So naturally the RAID-1 on disks C and D would hold the metadata for any
DRBD devices on partition 2 of disks A and B.

So if any 1 disk gets busy the metadata writes don't decrease the sequential
write performance.  With this configuration if I lose any one disk I just have
to rebuild 1 TB.

Without the RAID-1s the metadata would not be replicated and I'd have to
rebuild 1TB because of loss of data, and a 2nd TB because of loss of metadata.
 Alas, since I want to be able to have an arbitrary number of logical volumes
for virtual machines I can't presize the raid-1.





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