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

george young gry at ll.mit.edu
Thu Jan 22 17:01:58 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.

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
> 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?
> 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?

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...

-- George
"Are the gods not just?"  "Oh no, child.
What would become of us if they were?" (CSL)

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