New connections: SAS and iSCSI HBAs


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Alacritech NICs can send and receive any kind of TCP/IP traffic because they don't natively support initiators, a prerequisite on cards classified as HBAs. An initiator is the software that negotiates the connection between the controller and the storage target. Instead, Alacritech relies on the software initiator provided by Microsoft Windows to make this connection, while the QLogic iSCSI HBA uses a hardware initiator that's part of the HBA. Because of this design, QLogic HBAs support only iSCSI-based TCP/IP traffic and not other TCP/IP traffic like SMTP or FTP.

Compared to normal NICs, iSCSI cards from either vendor will deliver a quantifiable performance increase when doing iSCSI processing. Chris Sims, a network engineer with Clayton County Water Authority in Morrow, GA, found that using a QLogic QLA4052C iSCSI HBA on his VMware ESX Servers connected to an EqualLogic Inc. PS300 storage array resulted in a 10% to 20% improvement in host CPU performance vs. normal NIC cards.

Alacritech NICs and QLogic iSCSI HBAs each handle this TCP offload differently. QLogic iSCSI HBAs offload and handle all TCP functions natively, while Alacritech NICs leave a small percentage (generally 1% or less) of the TCP processing--error handling and connection management--to the host CPU.

To improve performance, Alacritech NICs handle the

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TCP offload by performing two functions similar to the way QLogic iSCSI HBAs handle the chore. First, Alacritech NICs use an ASIC to expedite the handling of TCP instructions. QLogic iSCSI HBAs use a hardware logic-based TCP/IP offload engine with two embedded specialized processors that mainly expedite the iSCSI offload.

The second way Alacritech NICs speed data movement is by moving incoming data directly into the application buffer. Traditional NICs receive data into their buffers, which the host CPU then copies to a network buffer reserved in the host's memory. The host CPU then removes the iSCSI packaging around the data and moves the data from the host's network buffer into the host's reserved application buffer where the application can access it.

QLogic iSCSI HBAs don't eliminate these two data moves; however, they eliminate some of the host CPU overhead because the data is already SCSI block data on the iSCSI HBA and can be moved into the application buffer without requiring the host CPU to remove the iSCSI packaging. The ASIC on Alacritech NICs eliminates all of these steps that the host CPU must normally take by moving the data directly from the network card buffer to the buffer on the host reserved for the application. More HBA vendors are beginning to use ASICs to expedite the processing of TCP traffic. QLogic recently announced that it intends to license Alacritech TCP offload patents and use Alacritech ASICs in a future generation of QLogic 10Gb/sec TOE iSCSI HBAs.

This was first published in April 2007

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