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I have a server with HP NC523 dual 10GbE network card and HP MSA1040 storage with dual 10GbE iSCSI-controller.

Both ports on NC523 are configured as a bond. The ports on MSA have different IP-addresses.

I want to configure this system to use 20Gb bandwidth (i.e bond on linux server and multipath on MSA). No switch should be used, just a direct connection.

If both ports of MSA are connected with fiber optic cables, only one port is reachable with ping (and presumable for that reason I can't initialize multipath). I cross-checked all the hardware components and all of it works in each possible combination as long only one line connects server with MSA.

Can somebody suggest me solution to this problem?

p.s.

I have another similar server with a "normal" quad 1 GbE copper network card and an older HP P2000 storage with quad 1GbE iSCSI controller. Analogous configuration works here without any problem. The four interfaces on server are bound to a single bond and each port of iSCSI-storage can be pinged and also multipath works. Both machines are running with CentOS 7.

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Never use LACP or any other kind of network aggregation for iSCSI networks unless it was required by your SAN vendor. Use MPIO (Multipathing) instead. 

Generally speaking, teaming creates a network overhead by adding an extra text string into each Ethernet frame.    Usually, nothing bad happens during the low workload or when teaming is used along with NAS protocols (NFS/SMB) since the number of Ethernet frames per second is not really high.    In case of iSCSI traffic which, essentially, turns into block level access, the number of frames per second may be really high especially when smaller 4k/8k access patterns are being used. When iSCSI networks are teamed, LACP driver processes each frame which leads to an extra CPU load and increased latency.

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You're not supposed to use LACP "bonding" with iSCSI. This is a situation where you should be using MPIO multipath.

Regardless, you won't be able to achieve 20Gbps speeds with this setup (one server->storage connection). That's not how bonding or MPIO work.

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