By Jeff Braunstein, Director of Product Management, Spectra
We’ve seen a new category of software being referred to as Storage Management Software (SMS). Some have questioned whether this is something new or if it is just another offering of HSM (Hierarchical Storage Management). While it may seem that HSM and SMS are interchangeable, there are clear differences between the two categories. Yes, both solution types are designed to keep frequently accessed data on high performance storage (usually the most expensive), while moving inactive data to other types of storage (usually much less expensive). However, how these solutions accomplish this process is very different.
The Importance of Transparency
A common vocabulary is critical to communication. To that end, let’s lay out a few terms vital to this discussion: Stub Files, Symbolic Links and HTML Links. These are the “bread crumbs” left behind when a file is moved. This is how the application or user is going to get the file back as transparently as possible. The level of transparency differs based on methodology and is a key differentiator between HSMs and Storage Management Software.
HSMs map nearline storage to appear to be online, including tape storage and the “cold” tiers of cloud storage. This is problematic given the sometimes long latency of nearline storage. Applications will time out if they don’t get the data requested in the time they expect to get it. To achieve this, HSMs typically use stub files (along with filter drivers, but we don’t need to go that deep). The stub file looks like the original file and often contains the first bytes of the original file. The stub file can respond to the read request while the HSM gets the rest of the moved file and brings it back. HSMs become part of the file system, usually have kernel code, are very operating system (OS) dependent, have to be upgraded with the operating system, and offer no ability for the application or the user to see that a file has been moved.
HSMs don’t play well in all environments. However, they work well in High Performance Computing (HPC) environments where the operating systems are more “time-out tolerant” or “HSM aware.” Successful HSMs like HPSS and DMF, are extremely effective. They are also expensive, complex and require lots of resources – but when you need them, nothing else will do.
Solutions in the category of Storage Management Software, such as Spectra’s StorCycle, require less budget, headcount and infrastructure, and sit well outside of the file system. There are some exceptions, but this is a good categorization of modern Storage Management Software. This type of software is less complex and more compatible with a large range of applications and use cases. Symbolic links and/or HTML links are more likely to be used to find the moved data. These links work quite differently from stub files as well as from each other.
By leaving a symbolic link in place of the original file, a read can simply be redirected to where the file has been moved. This works great when moving infrequently accessed data off of primary storage (high-speed disk or SSD) to a lower tier of storage like NAS disk. Most applications can tolerate the small increase in latency. However, this methodology does not work well with tape or low response level cloud. That’s where the HTML links come in.
Solutions that support HTML links are created specifically to support storage mediums with longer latency, such as tape or the cold tiers of cloud. When an HTML link is left in place of the migrated file, the user is presented with an HTML page which states that the file has been archived, gives information about when, where and how this was done, and allows the user to start a restore from tape or recall the file from cloud without having to contact IT. So transparency exists, but as noted earlier, it is a significantly different level of transparency (with HTML links) and offers distinct advantages over those derived from HSM solutions.
Protecting Data
The final differentiator between HSMs and Storage Management Software relates to data protection. Storage Management Software is designed for both primary storage offload and data preservation. That has not been the focus of HSM solutions. Storage Management Software allows multiple copies of the original file to be created and stored in multiple locations and on multiple storage types during the data migration process. In addition to moving infrequently accessed files from expensive primary storage to lower cost NAS, savvy users can send a copy to an on-premise tape library, to offsite tape storage or to the cloud for disaster recovery.
In this way, Storage Management Software can decrease the size of the Primary Tier of storage, increasing overall performance, decreasing backup windows, and eliminating more primary storage purchases, all while assuring long-term protection of and perpetual access to the less active data via multiple copies to multiple mediums.
While the lines are often blurred between HSM and Storage Management Software, as well as between individual solutions that fall into each category, there are numerous differences that are important to identify and consider when implementing a data tiering solution for the specific needs of an organization.
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About the Author
Jeff Braunstein, Director, Product Management
As Spectra’s Director of Product Management, Jeff is responsible for all Spectra disk-based storage solutions, including the entire BlackPearl® product portfolio, as well as Spectra’s new storage management software solution. Jeff has been with Spectra for 15 years and has held positions in the areas of Product Management, Marketing Operations and Business Systems Management. He earned an M.S. and B.S. degree in Engineering from the University of California, Davis.
