Shared DAS

Code Name "Thumper" by richardmasoner (cc) (from Flickr)
Code Name "Thumper" by richardmasoner (cc) (from Flickr)

An announcement this week by VMware on their vSphere  5 Virtual Storage Appliance has brought back the concept of shared DAS (see vSphere 5 storage announcements).

Over the years, there have been a few products, such as Seanodes and Condor Storage (may not exist now) that have tried to make a market out of sharing DAS across a cluster of servers.

Arguably, Hadoop HDFS (see Hadoop – part 1), Amazon S3/cloud storage services and most scale out NAS systems all support similar capabilities. Such systems consist of a number of servers with direct attached storage, accessible by other servers or the Internet as one large, contiguous storage/file system address space.

Why share DAS? The simple fact is that DAS is cheap, its capacity is increasing, and it’s ubiquitous.

Shared DAS system capabilities

VMware has limited their DAS virtual storage appliance to a 3 ESX node environment, possibly lot’s of reasons for this.  But there is no such restriction for Seanode Exanode clusters.

On the other hand, VMware has specifically targeted SMB data centers for this facility.  In contrast, Seanodes has focused on both HPC and SMB markets for their shared internal storage which provides support for a virtual SAN on Linux, VMware ESX, and Windows Server operating systems.

Although VMware Virtual Storage Appliance and Seanodes do provide rudimentary SAN storage services, they do not supply advanced capabilities of enterprise storage such as point-in-time copies, replication, data reduction, etc.

But, some of these facilities are available outside their systems. For example, VMware with vSphere 5 will supports a host based replication service and has had for some time now software based snapshots. Also, similar services exist or can be purchased for Windows and presumably Linux.  Also, cloud storage providers have provided a smattering of these capabilities from the start in their offerings.


Although distributed DAS storage has the potential for high performance, it seems to me that these systems should perform poorer than an equivalent amount of processing power and storage in a dedicated storage array.  But my biases might be showing.

On the other hand, Hadoop and scale out NAS systems are capable of screaming performance when put together properly.  Recent SPECsfs2008 results for EMC Isilon scale out NAS system have demonstrated very high performance and Hadoops claim to fame is high performance analytics. But you have to throw a lot of nodes at the problem.


In the end, all it takes is software. Virtualizing servers, sharing DAS, and implementing advanced storage features, any of these can be done within software alone.

However, service levels, high availability and fault tolerance requirements have historically necessitated a physical separation between storage and compute services. Nonetheless, if you really need screaming application performance and software based fault tolerance/high availability will suffice, then distributed DAS systems with co-located applications like Hadoop or some scale out NAS systems are the only game in town.