[DRBD-user] DRBD with SSD primary, spindle drive secondary

Andrew Eross eross at locatrix.com
Fri Sep 21 12:11:53 CEST 2012

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

Hi guys,

I've been doing the pre-requisite Google research and I haven't reached a
conclusion, so thought I'd ask here. I have an experimental pair of
identical XenServers setup with DRBD running over a Gigabit cross-over
cable. The only difference is that the primary has a SSD and the secondary
is a normal spindle drive.

dd tests on the underlying hardware show:
* the spindle server is capable of writing at ~70MB/s
* the SSD server at ~250MB/s

If I put the primary into drbd standalone mode, I also get about ~250MB/s
when writing to the DRBD device.

When running in primary/secondary mode, however, we only get around the
~65MB/s mark, which makes perfect sense with protocol C.

I was expecting that if I switched to protocol A, I would be able to let
the SSD drive write at it's full speed (e.g. 250MB/s) only at the price of
the secondary potentially falling a little bit behind, however performance
is almost exactly the same with protocol A, B, or C at around 60-70MB/s.

I then tried combining that with "on-congestion pull-ahead;" to see if that
would allow the primary to write at full speed, but still, same result.

Is this simply not do-able for some reason to let the primary write at a
faster speed than the secondary?

Thank you!
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