Physically, a Unified Storage Device (USD) is a controller and one or more shelves of disks (the low-end systems, such as the FAS 2xx series, and the FAS 20xx series integrate the controllers into the shelf. The controller is connected to the shelf through a Fibre Channel or SAS link (which looks like a thin orange wire). Inside the controller is a nonvolatile RAM (NVRAM) card, also called NVMEM. This is battery-backed memory into which all writes are logged. If the power fails, uncommitted writes are replayed from the log when the server comes up.
Below is the picture of a FAS6070 controller. Controllers are made to fit into standard 19-inch racks. The FAS6070 contains two Intel CPUs and 9 slots for network and SAN connectivity, and so on.
Disk shelves also fit into the 19-inch racks. One controller can service many disk shelves. Below is the picture of a DS-14 disk shelf. It has room for 14 disk drives, and can hold several terabytes. Except for the NearStore product, which uses ATA drives, all disks are Fibre Channel disks. Fibre Channel is the traditional choice for high-performance RAID arrays, or SAS, an emerging contender in data center environments.
The USD's operating system, Data ONTAP, uses a file system called Write Anywhere File Layout (WAFL). This file system uses a patented technique of ensuring file system consistency that also provides for very efficient file system Snapshot copies. Snapshot copies are taken almost instantaneously, and use no extra space until more data is written.
USDs can be clustered together, in what is known as an active/active configuration, for high availability (HA). In addition to having a link between the USDs to keep their write caches (NVRAM) coherent, each has a Fibre Channel along with or without a SAS link to all the partner's disks. If one partner fails, or is intentionally taken down for an operating system upgrade or maintenance, the live partner continues to serve data from the down partner's disks to all clients of the down partner. The HA product is also used to perform nondisruptive upgrades.
Most systems are delivered preinstalled in system cabinets with the operating system installed. A fully configured cluster pair can serve 1,008 TB of SATA storage, suitable for the Nearline storage, at present. High-performance Fibre Channel storage is less dense, so the maximum is less than that in a high-performance configuration. The picture below is one of these petabyte units, with the controllers and disk shelves racked into cabinets.
There is also a midrange line of equipment, which looks a lot like the high-end gear. All of the midrange systems run the same operating system and file system, and have the same capabilities as the higher end models, but are constrained in terms of capacity and performance.
The FAS 250,above, is the legacy entry-level system. Instead of a least recently used (LRU), the card that the disk shelf uses to speak to the Fibre Channel loop, it has a miniature USD controller. If the unit becomes full, upgrading means trading to a higher-end USD controller, removing the old controller, and inserting an LRU. The FAS250 thus becomes one disk shelf in a larger system, with no need to copy data, and with minimal downtime. This top-to-bottom architectural cohesiveness is a great strength of the entire Unified Storage product line.
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FAS250r |
FAS2050 |
FAS2020 |
The FAS270 is the clustered version of an FAS250, with two USD controllers inside a single disk shelf. This configuration was much used. When running RAID-DP, and keeping a spare for each controller, a total of six out of 14 disks were consumed by parity and sparing. This leaves only four disks for data on each controller. Later versions of the product added a second disk shelf. The FAS2050, introduced in 2007 and shown above, features an InfiniBand connection between the controllers for NVMEM synchronization, and allows up to six disk shelves to be added. Its sibling, the FAS2020, is a 2U version of the product that supports up to 20 spindles.
All FAS, NearStore, and V-Series products are true network appliances. Clients and workstations are connected to them over a LAN or a SAN. To these clients and workstations, USDs appears like a Windows and UNIX file server, or another other SAN-attached array. This means installing a USD is a matter of plugging it in and configuring it. There are no drivers or additional software needed on the clients, because the device speaks standard protocols already understood by the clients.
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