RAID1 will only limit requests to 4K if the device beneath it is
non-contiguous - e.g. a striped array or LVM arrangement were consecutive
blocks might be on different devices.

how does it know if a device is non-contiguous? Is there a way to have a dm device "marked" like that or force md to use bigger requests?
Lets assume a host with about 20 raid1 devices consisting of dm-devices with the mentioned overhead - do you think that not splitting the requests could lead to a noticeable performance improvement?

Thanx for your help,
Ramon Schönborn
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