Chelsio S3 iSCSI Driver for Linux
Introduction
============
The Chelsio T3 ASIC based Adapters (S310, S320, S302, S304, Mezz cards, etc.
series of products) supports iSCSI acceleration and iSCSI Direct Data Placement
(DDP) where the hardware handles the expensive byte touching operations, such
as CRC computation and verification, and direct DMA to the final host memory
destination:
- iSCSI PDU digest generation and verification
On transmitting, Chelsio S3 h/w computes and inserts the Header and
Data digest into the PDUs.
On receiving, Chelsio S3 h/w computes and verifies the Header and
Data digest of the PDUs.
- Direct Data Placement (DDP)
S3 h/w can directly place the iSCSI Data-In or Data-Out PDU's
payload into pre-posted final destination host-memory buffers based
on the Initiator Task Tag (ITT) in Data-In or Target Task Tag (TTT)
in Data-Out PDUs.
- PDU Transmit and Recovery
On transmitting, S3 h/w accepts the complete PDU (header + data)
from the host driver, computes and inserts the digests, decomposes
the PDU into multiple TCP segments if necessary, and transmit all
the TCP segments onto the wire. It handles TCP retransmission if
needed.
On receving, S3 h/w recovers the iSCSI PDU by reassembling TCP
segments, separating the header and data, calculating and verifying
the digests, then forwards the header to the host. The payload data,
if possible, will be directly placed into the pre-posted host DDP
buffer. Otherwise, the payload data will be sent to the host too.
The cxgb3i driver interfaces with open-iscsi initiator and provides the iSCSI
acceleration through Chelsio hardware wherever applicable.
Using the cxgb3i Driver
=======================
The following steps need to be taken to accelerates the open-iscsi initiator:
1. Load the cxgb3i driver: "modprobe cxgb3i"
The cxgb3i module registers a new transport class "cxgb3i" with open-iscsi.
* in the case of recompiling the kernel, the cxgb3i selection is located at
Device Drivers
SCSI device support --->
[*] SCSI low-level drivers --->
<M> Chelsio S3xx iSCSI support
2. Create an interface file located under /etc/iscsi/ifaces/ for the new
transport class "cxgb3i".
The content of the file should be in the following format:
iface.transport_name = cxgb3i
iface.net_ifacename = <ethX>
iface.ipaddress = <iscsi ip address>
* if iface.ipaddress is specified, <iscsi ip address> needs to be either the
same as the ethX's ip address or an address on the same subnet. Make
sure the ip address is unique in the network.
3. edit /etc/iscsi/iscsid.conf
The default setting for MaxRecvDataSegmentLength (131072) is too big,
replace "node.conn[0].iscsi.MaxRecvDataSegmentLength" to be a value no
bigger than 15360 (for example 8192):
node.conn[0].iscsi.MaxRecvDataSegmentLength = 8192
* The login would fail for a normal session if MaxRecvDataSegmentLength is
too big. A error message in the format of
"cxgb3i: ERR! MaxRecvSegmentLength <X> too big. Need to be <= <Y>."
would be logged to dmesg.
4. To direct open-iscsi traffic to go through cxgb3i's accelerated path,
"-I <iface file name>" option needs to be specified with most of the
iscsiadm command. <iface file name> is the transport interface file created
in step 2.
