Shared serial-attached SCSI (SAS) has emerged as an option for smaller companies that are looking to reap some of the benefits of a storage area network (SAN) without the costs associated with iSCSI or Fibre Channel. Greg Schulz, founder and senior analyst with the Storage IO Group, talks about the pros and cons of shared SAS in this Q&A. An audio recording of his answers is also available below.Listen to the Shared SAS FAQ
Table of contents
>> What is shared serial-attached SCSI?
>> What are the benefits of shared serial-attached SCSI?
>> Is shared serial-attached SCSI affordable?
>> What are the drawbacks of shared serial-attached SCSI?
>> What are the major shared serial-attached SCSI vendors?
>> Who is using shared serial-attached SCSI today?
SAS is an interface for attaching storage to servers. Also, SAS is a type of disk drive. Shared SAS is a way to attach a storage array to more than one server.
So, there is shared SAS, but there is also (DAS), meaning storage can be directly attached to two or more servers with SAS cables.
You either have storage that is directly attached and internal to the server, or you have storage that is directly attached and external and shared. Normally, when you think about shared storage, it's a SAN. Shared SAS is an alternative.
The benefits include simplified management, simplified backup and improved utilization. Virtual servers, whether you are using VMware, Hyper-V or Xen, require shared storage. If you are going to use VMotion for live migration of a virtual machine from one physical machine to another, those physical machines require shared storage.
In general, yes, it should be cheaper. However, it is limited in distance. But most servers today come with SAS on the motherboard, so there is the potential that you won't need a SAS adapter.
Depending on the implementation, shared SAS storage from companies such as Dell, HP, IBM, Oracle/Sun and others has the potential to be more affordable than iSCSI at low-end, small environments.
The biggest drawback is that your server either must support SAS or you need to purchase a SAS adapter. It's like iSCSI, you either have that 1 Gigabit Ethernet NIC built in or you have to buy it. Other than that, the biggest drawback is concerning distance. If you are looking to build out within tens of meters within your data center, you ought to be looking at something else. If you are looking to network within a cabinet or between two adjacent cabinets, shared SAS could be a candidate. Likewise, if you are looking to support 10, 20, 30 servers, you should be looking at iSCSI or Fibre Channel.
You don't hear about SAS from NetApp or EMC. You don't hear about SAS from vendors whose focus is on NAS or iSCSI. Why? These companies have a different value proposition and different approach. They're using SAS as internal storage in their systems. Dell and HP are the ones you hear about.
HP has really taken the lead with what they are doing with SAS at the low-end. Their MSA2000 and X series use SAS drives and the MSA2000 gives you the choice of an iSCSI, Fibre Channel or SAS interconnect. Even in its larger, scale-out systems like the X9000, HP is leveraging SAS internally. It is putting SAS in servers on the server board, same thing with Dell and IBM. It's really the server vendors that are taking the more aggressive approach with SAS.
There are two categories. There's the smaller company that has a smaller number of servers -- maybe two up to about eight servers depending on the configuration. It might be an environment in which iSCSI is too expensive.
The other scenario is a workgroup or department within a larger company with a small number of servers looking for a clustered, high-availability environment.
This was first published in July 2010