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The cost and technicalities of connecting SANs

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Bits & Bytes: Connectivity between storage area networks is in a period of transition. 1Gig and 2Gig is here today and 4 and 10 Gig is lurking right around the corner. For backup purposes, increased bandwidth could be the tonic that could help shorten your backup windows. But, are the costs for these technologies worth it? Christopher Poelker explores this question. Question: Fibre Channel supports 1G/2G today and 4G is on the horizon...

and people are also talking about 10G interfaces. iSCSI supports GigE and soon 10GigE will be here. Companies are also looking at extending their SAN networks over MANs/WANs.

Here are my questions:

1. How widely are the GigE and 10GigE connections available for MAN/WAN connectivity?

2. How do the GigE/10GigE service (price) compare to DWDM/dark fiber for MAN and SONET/ATM for WAN connectivity?

3. Even though FC and iSCSI support GigE interfaces, do these storage applications require dedicated 1G or more bandwidth at all times?

Once the data sources have been mirrored once, I would assume that the subsequent data syncs will be incremental and not so huge. So the bandwidth required will be lot less. Can you comment on this please? I am asking because buying that big of a pipe from an SP will be very costly for any enterprise and thus make SAN extension over MAN/WAN very expensive. Are speeds of T1/T3/OC-3 or even OC-12c enough for most of the applications? Maybe not for synchronous mirroring applications but for all other applications those speeds will be enough.

Chris Poelker's response

The most expensive ticket item in connecting SANs together for replication is usually the link costs. And yes, they are recurrent costs and can be very expensive depending on the bandwidth and distance provided. I am finding more availability of leasable Gig-E links from a number of providers. You are correct in the fact that after an initial copy for data replication to a remote site, the updates will usually require much less bandwidth.

Links need to be defined for worst-case scenarios. Some data replication solutions need to do a full resync if the connections are lost long enough. If you lease only the minimum required bandwidth, then a full resync may take an unacceptable amount of time.

There are always trade-offs. Bandwidth vs. cost is a typical one. Most companies would LOVE to have high-speed links between all offices but the costs outweigh the business benefits. You have to make a business decision on what bandwidth you can afford vs. the payback of having that bandwidth available.

You can lease lines that are billed on bandwidth used. This way, when you need to do an initial copy, you will have the required bandwidth but as your data rate slows, so do the bills. For a comprehensive discussion on the different connectivity options and its associated costs and technical capabilities, you can download a very good white paper done by Andy Helland of LightSand.

Andy's paper includes up-to-date pricing for 2003 of the different network links and compares the technical and financial considerations of large-scale data transfers with IP-based connectivity vs. SONET. You may have to register on the site to get the free white paper.

Hope this helps.

Chris

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This was first published in August 2003

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