New connections: SAS and iSCSI HBAs


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Another potential concern is how SAS HBAs communicate with SAS and SATA disk drives. Although SAS HBAs provide the potential to connect SAS and SATA disk drives on the same channel, SAS and SATA disk drives have different interfaces and respond to commands from SAS HBAs differently.

Mark Miquelon, LSI Logic's director of HBA products, Storage Components Group, says that when an LSI Logic SAS HBA communicates with SAS disk drives, the SAS HBA can operate in full duplex mode, allowing it to send commands and receive data simultaneously. To drive multiple SAS disk drives at the same time, command queuing in the SAS HBA becomes a big factor. This allows an initiator to load a SAS disk drive with commands, close that connection, go on to the next drive and load that one with commands. "The individual SAS drives can then come back and request a new connection to the initiator when they have data or a response," says Miquelon. "This prevents the initiator from getting tied up while the drive is working on the command from the initiator."

However, SAS HBAs talk to SATA disk drives differently. Many SATA disk drives require a connection to remain in place at all times and therefore don't allow the disconnect/reconnect cycle permitted by SAS drives. These communications between SAS HBAs and SAS and SATA disk drives become more complicated with the introduction of a SAS expander between the HBA and drives.

A final concern that administrators need to address is interoperability

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among SAS HBAs, expanders and disk drives from different SAS vendors. Adaptec Inc., a SAS HBA vendor, recommends users proceed with caution when intermixing SAS devices from different vendors. "Users should view SAS as a single-vendor play for now and check compatibility matrixes before implementing different vendors' products in a single SAS domain," says Paul Vogt, Adaptec's director of product marketing.

SAS HBAs, iSCSI HBAs and NICs are beginning to make inroads into the traditional DAS and SAN domains of SCSI and FC. It's best to implement these products in small, controlled environments when existing technologies like SCSI and FC are either too slow or too expensive. With more storage devices such as real and virtual tape drives adding support for SAS and iSCSI, new specifications like SAS-2 in development and faster processors to handle the overhead associated with 10Gb/sec Ethernet networks, expect SAS and iSCSI HBAs to become increasingly popular alternatives to SCSI and FC in the coming year.

This was first published in April 2007

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