SAN over WAN connections offer enterprises real time or near real time remote data replication. This creates viable and rapid disaster recovery capabilities not otherwise available. Use WAN acceleration devices to dramatically improve the performance of these remote links.
TCP/IP and Remote Replication
The first note in this series discussed latency and bandwidth utilization in SAN over WAN environments. The problem for many remote connections is the use of the "chatty" TCP/IP protocol. Simply put, the protocol uses numerous acknowledgements, limited window sizes, and slow recovery mechanisms in order to ensure high reliability and guaranteed delivery of data. It does not ensure high performance.
Enterprise SAN vendors provide the software for synchronous or asynchronous remote backups over the TCP/IP WAN. For Fibre Channel (FC) SANs, a conversion is often done to Fibre Channel over IP (FCIP) before sending data. For the growing body of Internet SCSI (iSCSI) SANs, no conversion is needed. The standard backup packages know nothing of the WAN connectivity and leave any WAN optimization to the networking components.
Devices are available that can compress and optimize remote connections for all TCP/IP traffic. These should not be confused with Wide Area File Services (WAFS) products. WAFS appliances compress and optimize file-based transactions such as CIFS and NFS. SANs are block-oriented operations, and WAFS-specific appliances will not optimize this traffic.
The optimum performance for SAN over WAN can be gained by using dedicated circuits. However, that option is not financially viable for many small to mid-sized enterprises (SMEs). The alternative is to utilize WAN optimization devices that compress and encrypt the data for transport over the Internet.
SAN over WAN Vendors
For enterprises that have dedicated WAN links between sites, SAN-specific optimization products are available. The SAN vendors themselves offer devices providing optimization and protocol enhancements. These SAN extenders support both FC and iSCSI storage protocols.
IBM offers products from Cisco, Brocade, and ADVA Optical Networking for their SAN extensions over dedicated channels. Because these products control the dedicated WAN link, they provide extremely high utilization of the link. By streaming data without waiting for acknowledgements and compressing the data before transmission the delays introduced by TCP/IP are eliminated.
The latency introduced by the physical distance between sites remains unchanged and can still limit the geographic range for synchronous replication operations. However, the improvement in performance introduced by these products can extend synchronous SAN replication to hundreds of miles.
General purpose WAN optimization devices can be used for SAN replication over the Internet. These devices offer data compression, encryption, and some level of protocol optimization for all traffic between enterprise sites. There are a large number of vendors offering products in this area, including:
Like the dedicated WAN products, these devices resolve many of the performance issues inherent in TCP/IP protocols. By utilizing UDP or proprietary upper level protocols over the IP network, significant throughput increases can be realized. These devices offer rate limiting capabilities to ensure that other traffic continues to receive fair treatment.
Both of the SAN over WAN and general purpose WAN optimization devices require devices to be installed at each site that will be communicating with one another. Their optimization schemes for compression, caching, and protocol control are often proprietary and hardware-intensive.
Recommendations
- Use dedicated/leased WAN circuits for SAN over WAN replication wherever possible. These links and the associated optimization products use condensed protocols and can utilize nearly all of the available bandwidth.
- Internet replication works well for asynchronous backups. Using general purpose WAN optimization devices can provide improved performance over the Internet for both SAN replication and other site-to-site data.
- iSCSI SANs may not need any additional acceleration for asynchronous replication. iSCSI utilizes TCP/IP and can be sent over standard routers. However, the data must be encrypted before sending it over the Internet.
- Many FC SAN controllers provide FC to FCIP translation. Once the FC traffic is converted to FCIP, it can be transmitted over any TCP/IP network and, for asynchronous replication, may not need additional WAN optimization devices.
- Test the SAN replication products in the enterprise environment. Without actual end-to-end testing, the real world performance improvements of these devices on SAN replication cannot be calculated accurately. For synchronous operations it is critical to verify that the performance (both throughput and latency) will meet enterprise needs.
Bottom Line
Use WAN accelerators to dramatically improve the performance of SAN over WAN replication. This allows greater distances for synchronous replication and higher utilization of both dedicated WAN links and Internet connections.
