Software Libre

While running some tests in a client's environment, we've noticed reading from a partition of a multipath device was considerably slower than reading from its parent node:

[root@none]# dd if=mpath4 of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 8.92711 seconds, 120 MB/s

[root@none]# dd if=mpath4p1 of=/dev/null bs=1M count=1024 skip=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 17.5965 seconds, 61.0 MB/s

We asked to client support of a well-known GNU+Linux vendor, and they indicated that this behavior was "expected", since this kind of partitions were created by stacking a dm-linear device over the original multipath node. I wasn't satisfied by this answer, since AFAIK dm-linear only did a simple transposition of the original request over an specified offset (the beginning of the partition), so I decided to investigate a bit further on my own.