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

Lars Ellenberg Lars.Ellenberg at linbit.com
Thu Jan 22 17:38:44 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.

/ 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.

> > for [IDE|single physical spindle] or if it also badly affects RAID boxes
> > presenting as one disk?
> > 
> > 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.

> > I wanted to split the space up into partitions such that business functions
> > [cvs & project storage] would be less troubled by disk hogs home directories.
> > 
> > can or is it wise, to have drbd mirror the whole device sda and have
> > partitions in it?

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.

> Some RAID controllers allow you to configure multiple "virtual drives",
> i.e. if the array has enough physical drives, split them into two or
> three groups, each group a raid 5 or whatever.  This would avoid most of
> the "same spindle" performance problem.  You still have a possible
> bottleneck at the raid ctlr itself it it's not fast enough at calculating
> ecc data for writes.
> 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

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