[CentOS] bad iscsi performance after upgrade to CentOS 7.2

Mon Jun 20 09:15:03 UTC 2016
Ulrich Leodolter <ulrich.leodolter at obvsg.at>

hi all,

after i upgraded a physical server (SUN FIRE X4170) from CentOS 6.8 to 7.2
i am not able to get the same iSCSI read performance.

the server is connected to HP P2000 Storage via 2 x 1GbE Ethernet.

CentOS 6.8 gives me full read performance on raw iSCSI devices /dev/sdxx at 115MB/s.
CentOS 7.2 allows only 90-100MB/s, read performance varies and is not stable like for 6.8

the multipath performance on 7.2 is even worse.

CentOS 6.8 allows to read a full speed and stable 220MB/s.
CentOS 7.2 maximum was 140MB/s after i disabled readahead for multipath devices in tuned.

btw. i tried many recommended optimizations in 7.2, but i am not getting the raw iSCSI read
performance than on 6.8,  and on 6.8 i had no special tuning für iSCSI.

mtu 9000 is set and verified for both OS, other settings like network buffers are almost identical.

has anyone an idea why iSCSI read performance degraded in CentOS 7.2 ?

best regards