[DRBD-user] newbie Q: 'two drbd devices on the same spindle' and spindle is raid box

Todd Denniston Todd.Denniston at ssa.crane.navy.mil
Thu Jan 22 18:53:31 CET 2004

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

Lars Ellenberg wrote:
> / 2004-01-22 11:01:58 -0500
> \ george young:
> > On Thu, 22 Jan 2004 10:23:43 -0500
> > Todd Denniston <Todd.Denniston at ssa.crane.navy.mil> threw this fish to the penguins:
> >
> > > Hi,
> > > [drbd-0.6.10, Fedora Core release 1, 2 node]
> > > As I was beginning to create my drbd.conf, using the
> > > /usr/share/doc/drbd-0.6.10/drbd.conf example, I read the following comment:
> > >
> > >         # btw, don't do this.
> > >         # did you notice that in this example we have two drbd devices
> > >         # on the same spindle (hda)?  performance will be bad.  if you
> > >         # use several drbd devices, put them on different spindles;
> > >         # different channels/controllers won't be a bad idea for IDE.
> > >
> > > I have a raid box [Promise RM8K, RAID 5, ~750GB, SCSI], which presents itself
> > > as one disk, AKA spindle, and I wanted to know if the above note was mainly
> now, if it has more than one physical spindle, it _has_ more than
> one spindle. So the argument does not hold exactly.
> > > Is this because DRBD/EXT3 makes some assumptions about stacking up writes that
> > > are not true when presented with multiple partitions, or just that the seek
> > > times get amplified badly with drbd in protocol=C?
> it is mostly because of seektime killing your resynchronization performance.
> you can avoid that nowadays by putting the devices in different "sync-group"s.
> you cannot have partitions in drbd.  what you can do is have drbd as
> a LVM2-PV. but you should know why you are doing it before you try this.
> > Also keep in mind that you are mirroring a block device, not a file system.
> > DRBD doesn't know or care that you may only be using 20% of a given fs,
> > it will still sync the whole device's contents across the net when it
> > chooses.  Work out how long it would take to copy your entire 750GB
> > across your net...
> Yes. Don't mirror more than neccessary.  I'd suggest to have a LVM
> logical volume as the lower level device for drbd.  If you later
> figure you need more space, you can easily but *carefully* do
> lvextend, change the disk-size in drbd.conf, e2fsck -f, resize2fs,
> and be happy...
>         Lars Ellenberg
Thanks for the info, I want to re-state it to make sure I understand

If I had more disks in the array (currently only 5) and it handled presenting
multiple logical spindles, I MIGHT be better off doing it that way, however
just having a multiple physical disk RAID mitigates the issues (some).
With 'modern' drbd I can mitigate what was seen as the biggest issue, with
single spindle multiple drbd, by making sure each partition (or logical
volume) in a single array is in a different "sync-group".

And yes I did not want to have to sync the whole array if I could keep from
it, I even intend to have one partition read only 99% of the time because it
only needs to be updated when I update shared application binaries.

It is nice to hear that DRBD is working with LVM-2, it was something I had
thought about in passing, but as I don't yet know LVM's I am going to attempt
to put implementing that on a server, off to the next upgrade.

Todd Denniston
Crane Division, Naval Surface Warfare Center (NSWC Crane) 
Harnessing the Power of Technology for the Warfighter

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