[DRBD-user] DRBD Performance in disconnected mode ?

[Robert]

Hello list,

I'm evaluating DRBD performance in various scenarios right now - values 
as expected except for one value.

    100% random write (8k)

When doing 100% wandom write of 8k with i/o meter I get (repeatable):

218 TPS for the lower level LVM volume (/dev/vg_something/drbd0)
111 TPS with a DRBD device in connected mode (Protocol A over cross over 
GBIT)
99 TPS with the same DRBD device beeing disconnected (StandAlone 
Primary/unknown - manually disconnected the device with "drbdadm 
disconnect all")

The Filesystem is XFS
The Hardware is BBU supported 3Ware with 2x SATA HDD's and write cache 
enabled.
Config includes:
    no-disk-flushes;
    no-md-flushes;
    al-extents       1021;

I wonder:

- Why is DRBD in connetced mode faster than in disconnected mode ?
- Why is the lower level LVM device so much faster (mount options are 
the same) ?

Any hints ?

Regards,
Robert