The No. 1 rule to keep in mind when introducing a change in your primary data storage system is primum non nocere, or "First, do no harm." Data reduction techniques can help save money in disk systems, and power and cooling costs, but if by introducing these technologies you negatively impact the user experience, the benefits of data reduction may seem far less attractive.
The next challenge for data reduction in primary data storage is the expectation that space-saving ratios will be comparable to those achieved with data deduplication for backups. They won't. Most backup software creates enormous amounts of duplicate data, with multiple copies stored in multiple places. Although there are exceptions, that's not typically the case in primary storage. Many people feel that any reduction beyond 50% (a 2:1 reduction ratio) should be considered gravy. This is why most vendors of primary data reduction systems don't talk much about ratios; rather, they're more likely to cite reduction percentages. (For example, a 75% reduction in storage sounds a whole lot better than a 3:1 reduction ratio.)
If you're considering implementing data reduction technologies in primary data storage, the bottom line is this: Compared to deploying deduplication in a backup environment, the job is harder and the rewards are fewer. That's not to suggest you shouldn't consider primary storage data reduction technologies, but rather, you need to properly set expectations before making a commitment.
Primary storage data reduction technologies
The following are three primary storage data reduction technologies:
Compression. Compression technologies have been around for decades, but compression is typically used for data that's not accessed very much. That's because the act of compressing and uncompressing data can be a very CPU-intensive process that tends to slow down access to the data.
However, backup is one area of the data center where compression is widely used. Every modern tape drive is able to dynamically compress data during backups and uncompress data during restores. Not only does compression not slow down backups, it actually speeds them up. How is that possible? The secret is that the drives use a chip that can compress and uncompress at line speeds. By compressing the data by approximately 50%, it essentially halves the amount of data the tape drive has to write. Because the tape head is the bottleneck, compression actually increases the effective speed of the drive.
Compression systems for primary data storage use the same concept. Products such as Ocarina Networks' ECOsystem appliances and Storwize Inc.'s STN-2100 and STN-6000 appliances compress data as it's being stored and then uncompress it as it's being read. If they can do this at line speed, it shouldn't slow down write or read performance. They should also be able to reduce the amount of disk necessary to store files by between 30% and 75%, depending on the algorithms they use and the type of data they're compressing. The advantage of compression is that it's a very mature and well understood technology. The disadvantage is that it only finds patterns within a file and doesn't find patterns between files, therefore limiting its ability to reduce the size of data.
File-level deduplication. A system employing file-level deduplication examines the file system to see if two files are exactly identical. If it finds two identical files, one of them is replaced with a link to the other file. The advantage of this technique is that there should be no change in access times, as the file doesn't need to be decompressed or reassembled prior to being presented to the requester; it's simply two different links to the same data. The disadvantage of this approach is that it will obviously not achieve the same reduction rates as compression or sub-file-level deduplication.
Sub-file-level deduplication. Sub-file-level deduplication is very similar to the technology used in hash-based data deduplication systems for backup. It breaks all files down into segments or chunks, and then runs those chunks through a cryptographic hashing algorithm to create a numeric value that's then compared to the numeric value of every other chunk that has ever been seen by the deduplication system. If the hashes from two different chunks are the same, one of the chunks is discarded and replaced with a pointer to the other identical chunk.
Depending on the type of data, a sub-file-level deduplication system can reduce the size of data quite a bit. The most dramatic results using this technique are achieved with virtual system images, and especially virtual desktop images. It's not uncommon to achieve reductions of 75% to 90% in such environments. In other environments, the amount of reduction will be based on the degree to which users create duplicates of their own data. Some users, for example, save multiple versions of their files on their home directories. They get to a "good point" and save the file, and then save it a second time with a new name. This way, they know that no matter what they do, they can always revert to the previous version. But this practice can result in many versions of an individual file -- and users rarely go back and remove older file versions. In addition, many users download the same file as their coworkers and store it on their home directory. These activities are why sub-file-level deduplication works even within a typical user home directory.
The advantage of sub-file-level deduplication is that it will find duplicate patterns all over the place, no matter how the data has been saved. The disadvantage of this approach is that it works at the macro level as opposed to compression that works at the micro level. It might identify a redundant segment of 8 KB of data, for example, but a good compression algorithm might reduce the size of that segment to 4 KB. That's why some data reduction systems use compression in conjunction with some type of data deduplication.
Overall, each primary data storage reduction technique has its pros and cons, and none are better than the other. How you decide which technique is right for you comes down to your individual data storage environment and how these reduction techniques will fit in.
About this author: W. Curtis Preston (a.k.a. "Mr. Backup"), Executive Editor and Independent Backup Expert, has been singularly focused on data backup and recovery for more than 15 years. From starting as a backup admin at a $35 billion dollar credit card company to being one of the most sought-after consultants, writers and speakers in this space, it's hard to find someone more focused on recovering lost data. He is the webmaster of BackupCentral.com, the author of hundreds of articles, and the books "Backup and Recovery" and "Using SANs and NAS."
This was first published in August 2010