NAS heads: Gatekeepers for enterprise storage


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With a growing number of vendors providing NAS heads, here are some factors to weigh when selecting one.
Existing storage infrastructure. NAS heads from companies like Maxxan, Network Appliance (Net-App) and ONStor offer more flexibility by supporting a variety of storage vendors' products.
Number of Ethernet connections. For environments supporting a large number of active users, or those that need to keep network traffic separate for specific business reasons, consider high-end NAS head products from EMC or NetApp.
NAS head operating system. NAS operating systems must go well beyond the ability to just present files using either CIFS or NFS. Integration with existing network directory structures such as AD or LDAP is essential. Also, make sure the product either natively or optionally offers the ability to cluster NAS heads.
Storage management functions. All NAS heads include point-in-time snapshot functionality as part of their base operating system. Other commonly available options include mirrors, quota management and the ability to re-stripe data across back-end storage. Larger shops should consider NAS heads that include global name services, and ILM or policy management.
Protocol support. CIFS and NFS are givens for all NAS heads. iSCSI allows administrators to use NAS heads as a common SAN/NAS interface for some of their servers. NDMP support should be included for those who wish to offload some jobs from their backup server.
Must-have features
With all the developments related to NAS heads, certain features remain core to their basic functionality. Front-end Ethernet ports that provide host server connectivity have always been a staple. All vendors' NAS heads support at least two 1Gb/sec Ethernet ports with maximum configurations topping out at anywhere from four to 20. However, high-end models like EMC Corp.'s NS700G and Celerra CNS offer up to 32 and 168 Ethernet ports, respectively, while NetApp's GF920c and GF980c support 24 and 32 ports, respectively.

EMC says that when users consolidate hundreds of general-purpose file servers, they may need the flexibility to configure multiple copper and optical Ethernet ports. For example, if preserving isolation between subnets is a priority, additional ports will be required for the configuration. If bandwidth permits aggregating ports, fewer ports will be needed on the NAS head.

While NAS head vendors universally support CIFS and NFS versions 2 and 3, NetApp now supports version 4 of NFS. NFS version 4 adds an access control list (ACL) attribute that allows for specific user- and group-level access. Version 4 also offers a better client/server file locking and unlocking mechanism that allows files open for write to be closed under certain conditions if access to the file is requested by another client. Despite the appeal of these enhancements, they're just beginning to be used; NetApp reports that most of its customers interested in NFS version 4 are still evaluating the technology.

The ability to connect to back-end storage using FC ports enables scaling of a storage infrastructure as needed. Lorie Beam, director of information technology at Smith Anderson, a Raleigh, NC, law firm, chose to directly connect her EMC Centera NS600G to the back-end CX600 using eight FC ports. Beam eliminated the FC switches from her configuration by directly connecting the NS600G to the CX600. This kept costs down without hindering future growth. Using FC to connect to multiple tiers of storage and create storage pools is an increasingly popular way to get more mileage out of the storage deployed behind the NAS head.

For example, EMC's Celerra connects to its high-performance DMX arrays and Clariion CX arrays. To optimize these different types of storage capacity, third-party software from firms such as Arkivio Inc. or Enigma Data Systems (now part of Enigma Data Solutions) interacts with the Celerra FileMover API. This gives admins the ability to set policies that transparently move data between the different tiers of back-end storage.

TJ Klise, director of technology services at Methodist Medical Center in Peoria, IL, uses a Picture Archiving and Communication System (PACS) in conjunction with a NAS head from ONStor Inc. and IBM Corp./Tivoli Storage Manager software to automate the filing, aging and archiving of the medical center's imaging records. But despite the emergence of better policy management methods, quota management--a long-standing NAS head feature--remains popular. For example, Beam at Smith Anderson limits the amount of capacity of users' home directories to 350MB, but doesn't impose a limit on the shared document directory all users access. With all of the firm's legal documents numbered and indexed, she wants to ensure all documents are stored on the central file store for easy access, protection and recovery.

Snapshots and mirroring functionality are used increasingly for data protection. NetApp, with its Data Ontap operating system, and other vendors like Dell Inc., Hewlett-Packard (HP) Co. and Maxxan Systems Inc., which base their NAS head offerings on Windows Storage Server 2003, include free snapshot capabilities.

Raising the bar
The combination of CIFS, NFS and iSCSI protocols allows for the use of a NAS head for SAN/NAS convergence. While NetApp was one of the first to include iSCSI support in its NAS heads, a growing number of vendors added this functionality in 2004, including BlueArc Corp., EMC, HP and Maxxan.

HP says iSCSI adds flexibility and a wider level of application support to its HP ProLiant Storage Servers. With iSCSI, file systems and block-based volumes may be shared over the same Ethernet port with minimal performance degradation. While HP requires users to license the iSCSI Feature Pack for their ProLiant Storage Servers, it enhances the basic iSCSI protocol by including snapshot, clustering and direct backup features.

With nearly every array certified with the Windows operating system, storage administrators can break apart their storage and NAS head purchases to achieve their desired configuration. For example, Klise is running the medical center's ONStor 4420 NAS head with a back-end IBM FAStT 900 (now DS4500) with 21TB of SATA disk drives. Klise wasn't swayed by the reasoning that encouraged purchasing both the NAS head and arrays from one vendor to get better support and a fully certified environment.

With users like Klise putting different arrays behind appliance-based NAS heads, more vendors are expanding support of heterogeneous environments. But there are some notable exceptions: EMC and HP only support and certify the arrays they provide. NetApp's gFilers and ONStor's Bobcats certify and support their NAS heads for most major vendors' storage arrays. Maxxan's SG110 runs on Windows Storage Server and supports any array listed on Microsoft's compatibility matrix, while BlueArc's Titan SiliconServer supports arrays from Nexsan Technologies, Storage Technology Corp. and Xyratex Ltd.

However, supporting and certifying many vendors' arrays on the back end doesn't mean vendors' arrays support the NAS head operating systems on the front end. The certifications and support matrixes of back-end arrays are generally produced by NAS head vendors who may not go through the same rigorous testing that array vendors conduct. Risk-averse environments should stick to those configurations where both the NAS head and array vendor support and certify both ends of the solution.

This was first published in March 2005

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