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Tech Report: Content-addressed storage preferred for fixed-content storage

By  Jerome Wendt

19 Jun 2006 | SearchStorage.com

Content-addressed storage (CAS) safeguards retention data and prevents its alteration.

For most companies, fixed-content storage requirements are simple: Store the data securely, do it cheaply and provide fast access. With more data subject to external and internal audits, CAS products are becoming the preferred storage medium for the long-term protection of fixed content.

CAS products come in four different architectures:

1. The RAIN architecture is the predominant way vendors offer CAS hardware. Inexpensive servers or nodes with high-capacity disk drives are clustered together; software locks the data stored on each node. As growth occurs, more nodes are added to the RAIN cluster.

2. Network Appliance Inc. (NetApp) presents a network file system over an Ethernet connection on the front end while using WORM technology to lock the data down and data deduplication to optimize its capacity. The system accommodates growth by adding more disk capacity to NAS head configurations or allowing upgrades to higher capacity NAS filers. There's no way to move data from the NetApp disk to tape or optical media.

3. The HSM architecture offered by IBM Corp. allows applications to archive and retrieve data from the CAS system using APIs provided by the CAS software. IBM requires users to deploy its Tivoli Storage Manager (TSM) 5.3 for Data Retention software, which comes with its TotalStorage DR550. IBM's CAS product differs from the other CAS architectures because it doesn't use data deduplication or single-instance storage (SIS) by default, although users can deploy these technologies and use TSM to manage the data.

CAS products
Click here for a comprehensive list of CAS products (PDF).
4. Nexsan Technologies Ltd. offers a networked storage array architecture that includes CAS software as part of the array to manage data retention and ways to move data between disk, tape and optical.

There are trade-offs with each of these designs. Each one requires some type of software to classify and then move the data to and from the CAS device. Products using RAIN architectures--including Archivas Inc.'s Archiving Cluster (ArC), EMC Corp.'s Centera, Hewlett-Packard Co.'s (HP) StorageWorks Reference Information Storage System (RISS) and Permabit Inc.'s Permeon Compliance Store -- store files as objects. This introduces a new format for storing files whose long-term management costs and liabilities aren't yet well understood. File system approaches don't support SIS and require third-party products to manage file meta data. HSM architectures are based on a model that may not respond well with large data stores, while the network storage array approach has limitations on total disk capacity and the disk it will support.

CAS features

All CAS products deliver the following fixed-content storage requirements:

  • Data is accessible by content, not storage location
  • Scales economically
  • Manages large amounts of data
  • Guarantees data authenticity and security
  • Manages data-retention periods
  • Facilitates rapid data recall

To deliver these basic requirements, many CAS vendors use the RAIN grid architecture. A RAIN device is usually a commodity server (called a node) with internal SATA hard drives and vendor-supplied CAS software. The nodes in EMC's Centera and Permabit's Permeon RAIN architecture support two personalities: an access or portal node and a storage node. The access nodes are clustered and connected to the Ethernet network to receive and process either incoming files or requests for data using a number of network protocols. The access node identifies where the object is to be stored or where it resides, and then stores or retrieves the object from the storage node.

Protecting CAS data

Protecting data stored on CAS device presents an interesting dilemma for users because one of the primary purposes of a CAS product is to serve as a final resting place for data in an unchangeable format. HP deploys each of its SmartCell storage nodes in a mirrored configuration -- each node keeps a mirror of its partner's data. In the event that one-half of the mirror fails, a free node in the grid is allocated as a replacement partner to mirror the data; the failed mirror can then be repaired and reinserted in the grid.

EMC recommends replicating data to a second Centera at another site, but this is a costly alternative. To reduce the cost of a high-speed link between the two sites, the CBRM allows existing backup software to connect to the Centera and back up its data to tape using the NDMP protocol. CAS products from Bycast, IBM and allow you to copy each file or object to tape. However, this process reintroduces concerns about data integrity because someone needs to be responsible for moving the tape offsite and managing it in the long term.

You can also preserve the integrity of data or objects by balancing data across multiple storage nodes. Archivas Inc.' ArC and Permabit's Permeon Compliance Store use this approach to enable users to deploy nodes one at a time; this allows data to be distributed evenly across storage nodes. Each object is copied and stored on at least two different nodes to prevent data loss due to a node hardware failure.

Another consideration when evaluating each vendor's RAIN architecture implementations is the type of hardware to be used. Although each vendor uses off-the-shelf Intel server hardware to host its software, Archivas' ArC and Permabit's Permeon Compliance Store allow users to choose any vendor's brand of server, while CAS vendors such as EMC and HP require users to purchase server and storage hardware from them. HP only sells and certifies its ProLiant DL380 servers as nodes to support its StorageWorks RISS software.

Users with existing server hardware or server agreements may opt for Archivas' ArC or Permabit's Permeon Compliance Store because they run on any server vendor's hardware. For firms more concerned with deploying an end-to-end configuration sold and supported by a single vendor, choosing EMC or HP for the hardware and software in a preconfigured CAS product may be a better option.

The hashing algorithms a CAS product uses to create digital identifiers for each object is also important. Some hashing algorithms may be cracked or hacked over time; having the ability to upgrade the digital signature may therefore become more important. Caringo Inc.'s CAStor, a new CAS software product, lets users upgrade the hashing algorithm and digital signature as new ones become available.

Most RAIN architectures support only nodes with internal disk drives. Only Bycast Inc.'s StorageGrid and Caringo's CAStor let users deploy nodes that support different types of external storage and manage the placement of data on these different tiers of storage based on policies set by users.

A final concern is the protocols used to access the RAIN nodes. One way RAIN vendors circumvent the API problem is by presenting a mountable file system to the operating system (OS) and allowing apps to use the more common NFS and CIFS protocols to store and retrieve data. Most RAIN vendors, including Archivas, Bycast, HP and Permabit, support this configuration, and even EMC is jumping on the bandwagon.

NetApp's NAS products use file systems, but they support CAS in a slightly different manner. By using SnapLock (an optional WORM feature) with the Data Ontap OS that comes standard with all NetApp filers, and its new Advanced Single Instance Storage (ASIS) feature, users can lock down data and optimize storage capacity on filers. The main drawback of file-system architectures is that they require either a separate appliance or third-party software such as Open Text or FileNet to classify each file, create and store meta data, and manage the file's data-retention periods and user access permissions.

Pros and cons of RAIN products
Click here for a comprehensive list of the pros and cons of RAIN products (PDF).
IBM prepackages its TotalStorage DR550 with TSM for Data Retention software to enable apps to classify and manage data. (Shops already using TSM can host the Data Retention component on an existing TSM server.) For small- to medium-sized firms, IBM offers DR550 Express, which also ships with the TSM software, but supports only internal disk drives with an option for tape vs. the DR550 that supports external disk and tape and is available in clustered configurations. TSM is required to manage data placement, retention and security policies; all host apps will need to support TSM's APIs to store and retrieve data.

IBM prepackages its TotalStorage DR550 with TSM for Data Retention software to enable apps to classify and manage data. (Shops already using TSM can host the Data Retention component on an existing TSM server.) For small- andto midedium-sized firms, IBM offers DR550 Express, which also ships with the TSM software, but supports only internal disk drives with an option for tape vs. the DR550 that supports external disk and tape and is available in clustered configurations. TSM is required to manage data placement, retention and security policies; all host apps will need to support TSM's APIs to store and retrieve data.

Click here for more on content-addressed storage and fixed-content.

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