New connections: SAS and iSCSI HBAs


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Why SCSI won't go away

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Multiple factors are driving companies to move from SCSI to serial-attached SCSI (SAS) connectivity for their internal and DAS requirements. EMC Corp., for example, no longer offers SCSI front-end connectivity for its Symmetrix and Clariion storage arrays, and a number of storage vendors plan to end SCSI connectivity on their disk drives sometime in 2008. SAS host bus adapters (HBAs) also offer significant performance benefits over SCSI, supporting up to eight separate channels that operate at 3Gb/sec as opposed to 320MB/sec on a shared parallel SCSI bus.

Despite these limitations, SCSI HBAs remain a necessary component in enterprises. Key reasons SCSI HBAs will stick around include:
  • They're still needed for tape drives and other types of mass storage devices. Most vendors' SAS tape drives are still in development or just coming to market.
  • Parallel SCSI HBA components are mature and work.
  • Some older storage devices use only SCSI interfaces.

The need for speed
The need for higher performance drove Tim Bolden, president of Cary, NC-based iGlass Networks, to adopt SAS HBAs. His primary application server using a SCSI HBA and DAS was often I/O bound with queue depths staying at or exceeding five during peak times, which resulted in application slowdowns. This problem was especially visible because iGlass Networks provides real-time, outsourced network-monitoring solutions to its customers and the delays in processing the incoming alerts were encroaching on customer service-level agreements (SLAs).

Bolden was initially tempted to split the application and run it on four different servers to meet customer response time requirements. Instead, he purchased a new four-channel LSI Logic LSISAS3442E-R HBA along with Solid Access Technologies LLC's USSD 200SAS solid-state disk (SSD) array. Although the USSD 200SAS array played a big role in delivering improved application performance, the LSI Logic SAS HBA gave Bolden's application server four separate, dedicated 3Gb/sec paths to the disk on the USSD 200SAS array vs. the one shared 320MB/sec path he previously had to his SCSI-attached array. "By using a SAS HBA on the server I already had, my queue depth times [were reduced] to one on my existing server," says Bolden. This allowed him to "meet customer SLAs and avoid buying four new servers."

iSCSI HBAs can also improve I/O performance on Ethernet LANs. Both Alacritech network interface cards (NICs) and QLogic iSCSI HBAs include a TCP/IP offload engine (TOE) to handle the processing of TCP/IP and iSCSI traffic, although there are substantial differences between iSCSI cards.

Alacritech offers two types of iSCSI NICs--TOE NICs (TNICs) and iSCSI accelerators. Its SEN2000 and SEN2100 TNICs offload the entire TCP/IP stack, including iSCSI traffic from the server CPU onto the HBA, allowing them to accelerate all TCP traffic. This includes iSCSI as well as NFS or CIFS. However, Alacritech's SES2000 and SES2100 NICs offload only iSCSI traffic and leave behind all other TCP traffic for the server CPU to manage.

This was first published in April 2007

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