SNIA on STORAGE Evaluation criteria for data de-dupe How does it work? What are length or variable object length de-duplication, local ensure against hash collisions. Although concern over or remote de-duplication, inline or post-processing, and hash collisions is often raised, depending upon the hash de-duplicated or original format data protection. algorithm used and the system design, the probability the different implementation of a hash collision may actually be orders of magnitude Use of hashes less than the probability of an undetected disk read error methods? And what are the key Data de-duplication begins with a comparison of two returning corrupt data. data objects. It would be impractical (and very arduous) to scan an entire data volume for duplicate objects each Indexing evaluation criteria? time a new object was written to that volume. For that Once duplicate objects have been identified (and option- reason, de-duplication systems create relatively small ally validated), removal of the duplicate object can com- By Larry Freeman, Rory Bolt, mence. There are varying methods that systems employ when modifying their data pointer structures. However, and Tom Sas De-duplication pointers all forms of this indexing fall into four broad categories: Catalog-based indexing—A catalog of hash values is Reference pointers for duplicated objects used only to identify candidates for de-duplication. A DATA DE-DUPLICATION IS THE PROCESS separate process modifies the data pointers accordingly. of eliminating redundant copies of data. The advantage of catalog-based de-duplication is that The term “data de-duplication” was coined the catalog is only utilized to identify duplicate objects by database administrators many years ago as a way of and is not accessed during the actual reading or writing describing the process of removing duplicate database of the de-duplicated data objects; that task is handled records after two databases had been merged. via the normal file-system data structure. De-duplicated reference pointers Today the original definition of de-duplication has Lookup table-based indexing—Extends the function- been expanded. In the context of storage, de-duplication ality of the hash catalog to also contain a hash lookup refers to any algorithm that searches for duplicate data table to index the de-duplicated object’s “parent” data objects (e.g., blocks, chunks, files) and stores only a sin- pointer. The advantage of a lookup table is that it can be gle copy of those objects. The user benefits are clear: used on file systems that do not support multiple block ■ Reduces the space needed to store data; and referencing; a single data object can be stored and “refer- ■ Increases the available space to retain data for enced” many times via the lookup table. Lookup tables longer periods of time. hash values for each new object to identify potential may also be used within systems that provide block-level duplicate0712ISfreemanf1 data. services instead of file systems. How it works A hash value, also called a digital fingerprint or digi- Content-addressable store, or CAS-based, indexing—The Regardless of operating system, application, or file-system tal signature, is a small number generated from a larger hash value, or digital signature of the data object itself, type, all data objects are written to a storage system using string of data. Hash values are generated by a mathemat- may be used by itself or in combination with additional a data reference pointer, without which data could not ical formula in such a way that it is extremely unlikely metadata as the data pointer. In a content-addressable be located or retrieved. In traditional (non-de-duplicat- (but not impossible) for two non-identical data objects store (CAS), the storage location is determined by the ed) file systems, data objects are stored without regard to to produce the same hash value. In the event that two data being stored. Advantages of CAS-based indexing any similarity with other objects in the same file system. non-identical objects do map to the same hash value, include inherent single instancing/de-duplication, as well Identifying duplicate objects and redirecting reference this is termed a “hash collision.” pointers form the basis of the de-duplication algorithm. Due to the growing interest in data de-duplication and As shown in the figure, referencing several identical Evaluation criteria space reduction solutions, the SNIA DMF Data Protection Initiative has recently been tasked with forming a Spe- objects with a single “master” object allows the space Understanding a system’s use of hashes is an important cial Interest Group (SIG) focusing on this topic. This is the normally occupied by the duplicate objects to be “given criterion when you are evaluating de-duplication. If the first in a series of publications from SNIA on the topic of back” to the storage system. technology depends solely on hashes to determine if de-duplication and space reduction. The mission of the two objects are identical, then there is the possibility, DDSR SIG is to bring together a core group of companies Design considerations however remote, that hash collisions could occur and that will work together to publicize the benefits of data de-duplication and space savings technologies. Anyone Given the fact that all de-duplication technologies must some of the data referencing the object that produced interested in participating can help form the direction and identify duplicate data and support some form of ref- the collision will be corrupt. Certain government regu- ultimate success of the group. Find out more at erencing, there is a surprising variety of implementa- lations may require you to perform secondary data ob- www.snia-dmf.org/dpi. tions, including the use of hashes, indexing, fixed object ject validation after the hash compare has completed to 2 6 INFOSTOR www.infostor.com DECEMBER 2007 SNIA on STORAGE De-duplication ratio and storage savings as enhanced data integrity capabilities and with the application being de-duplicated the ability to leverage grid-based storage Number of backups rather than any technical advantages/ 8 16 20 architectures. Although CAS systems are disadvantages. inherently object-based, file-system seman- When performing data backups, the tics can be implemented above the CAS. user’s primary objective is the completion 99.5% Application-aware indexing—Differs 99% of backups within an allowed time win- 98% from other indexing methods in that it 96.7% 97.5% dow. For LAN- and WAN-based backups, 80% 95% looks at data as objects. Unlike hashing Acceptable remote inline de-duplication may provide or byte-level comparisons, application- 75% de-duplication ratio the best performance. For direct-attached (backup data) aware indexing finds duplication in ap- 67% and SAN-based backup, an assessment 50% plication-specific byte streams. As the 20% 43% should be made to determine which ap- name implies, this approach compares Acceptable proach works best. Either may be appropri- de-duplication ratio like objects (such as Excel documents to (non-backup data) ate, depending on data type and volume. If Excel documents) and has awareness of 1.25 1.5:1 1.75:1 2:1 3:1 4:1 5:1 10:1 20:1 30:1 40:1 50:1 100:1 200:1 post-processing de-duplication is deployed, the data structure of these formats. De-duplication ratio (n:1) users should ensure there is adequate time of variable object size de-duplication is is sometimes referred to as source de- between backup sessions to complete the Evaluation criteria that it allows duplicate data to be recog- duplication in the backup market. de-duplication post-process. De-duplication indexing is an important nized even if it has been logically shifted With general applications, the cost of consideration in technology evaluation, with0712ISfreemanf2 respect to physical block boundar- Evaluation criteria additional storage needed by post-process- particularly when it comes to resiliency ies. This can result in much better data The advantage of local de-duplication is ing needs to be weighed against the cost of design. When indexing data objects, de-duplication ratios. total application transparency and in- of system resources and the performance the index itself could become a single teroperability; however it doesn’t address of inline de-duplication to determine the point of failure. Evaluation criteria remote or distributed systems or bottle- best fit for an environment. It is important to understand what, if Fixed object length de-duplication offers necks in networks. Although it requires any, single points of failure are present in processing advantages and performs well a specialized agent or API, remote de- De-duplicated or original a de-duplication system. It is equally im- in both structured data environments duplication offers tremendous potential format data protection portant to understand what measures are (e.g., databases) and in environments for both network bandwidth savings and As is the case with all corporate data used to protect these single points of fail- where data is only appended to files. In application performance. systems, de-duplicating storage systems ure to minimize the risk of data loss. unstructured data environments such need to be protected against data loss. Another indexing consideration is as file servers, variable object length de- Inline or post-processing De-duplicating systems vary with respect the speed of the index. An inordinate duplication is able to recognize data that Another design distinction is when to to their approach to data protection. amount of time should not be required has shifted position as the result of edits perform de-duplication. Again, there are When protecting a de-duplicated sys- to store and retrieve data objects, even to a file.