iSCSI SAN « Storage Strategies Now

Product Test: The Drobo B800i iSCSI SAN

By Patrick Corrigan, senior analyst

Recently, SSG-NOW tested the Drobo B800i, an iSCSI SAN for small and mid-sized businesses. By way of background, Drobo, formerly Data Robotics (http://www.drobo.com), makes a family of multi-drive external storage devices for both servers and workstations under the Drobo brand name. Drobo devices have between four and twelve drive bays and, depending on model, support USB (2.0v and 3.), FireWire 800, gigabit Ethernet and iSCSI connections. All units use SATA drives, which are hot-swappable. Drobo’s BeyondRAID technology (described below) allows drives to be added, swapped or even removed as needed, and drive of differing sizes can be used. Depending on configuration, the number and capacity of installed drives, a Drobo unit can survive a failure or removal of either one or two drives concurrently. After a drive failure, the Drobo will reconfigure itself to return to a protected state. After reconfiguration, given sufficient disk space, a Drobo could survive another drive failure or removal even if the previously-failed had not been replaced. As capacity requirements grow, smaller drives can be replaced with larger drives. All Drobo devices support standard 3.5″ SATA drives of any capacity, including 3TB drives. As larger drives are available they should be supported, as well. The Drobo drive bays accept bare, carrier-less drives. A lever at the bottom of each bay ejects a drive or allows a drive to be quickly and easily installed. LEDs on the front of the Drobo show the status of the array and each drive.

All Drobo devices use Drobo’s proprietary BeyondRAID technology. With traditional RAID technologies you must define the protection level (RAID level) and the number of drives in the array. In addition, all drives must have the same data size. Once the parameters are set they can’t be changed without removing the data and manually rebuilding the array. Different sized drives can be used, but the data size is limited by the capacity of the smallest drive. With BeyondRAID, depending on the Drobo version, provides either single-drive or dual-drive protection. Four- and five-bay Drobos’ provide single-drive protection, which allows a single drive to fail without losing data. Eight- and 12-drive Drobos’ also provide the option of dual-drive protection, which allows two drives to fail without losing data. You can switch between single-drive and dual-drive protection pretty much at will.

Drobos’ allow the use of different sized drives while efficiently using the capacity of all drives in the array. Drives can be added and, given enough free disk space, removed pretty much at will, allowing capacity to grow or shrink while the array is in service. BeyondRAID also provides thin provisioning, meaning volume sizes can be defined that exceed currently-available disk space, and disk capacity can be increased as required without having to redefine volumes. Four- and five-bay Drobos’ create volumes automatically, and volume size is determined by the host system’s operating system. Eight- and 12-bay Drobos’ allow for user-controlled volume creation. Unlike traditional volumes that require pre-defined disk space, Drobo’s “Smart Volumes” are thin provisioned. Volumes pull storage capacity from a common pool as needed, so that multiple volumes can be defined that exceed actual available disk capacity. The disk space used by a volume, other than a slight bit of space used for volume definition, is only the space used by the data stored in that volume. Since volumes are virtual, if a volume is deleted, or if data is deleted from a volume, the space used is returned to the common pool.

As stated earlier, a Drobo can be configured to support concurrent failure of one or two drives. Given sufficient disk space you can switch between single or dual drive protection at any time. With single-drive protection Drobo reserves space approximately equal to the largest drive for protection. With dual-drive protection it reserves space approximately equal to the largest two drives. Drobo provides a capacity calculator (http://www.drobo.com/resources/calculator.php) to estimate the amount actual storage available based on the capacity of the disks in use.

When a drive fails or is removed the Drobo reconfigures itself around the currently working drives. When the failed drive is replaced the Drobo again reconfigures itself based on the new drive capacity. Drobo writes a unique identifier on each drive, so drives can be placed in any empty drive bay in any order. If a component in the Drobo fails, the drives can be removed and put in another Drobo, again in any order, and the array will continue to operate without data loss.

Drobo supplies a GUI management utility called Drobo Dashboard, which runs on Windows and Mac. Drobo Dashboard allows you to configure and manage the array. Although Drobo has limited Linux support there is currently no Linux version of Drobo Dashboard, so setup for Linux must be done using standard Linux utilities.

Volume sizes are limited by the operating systems being used. For example, Windows Server 2003 (and later) support volumes up to 16TB, while XP supports volumes up to 2TB. Because Drobo allows for thin provisioning, you can create volumes larger than the existing disk space and then add capacity as needed. Your computer’s operating system will display the defined volume size but Drobo Dashboard will allow you to monitor actual available disk space. Both Drobo Dashboard and drive status indicator LEDs on the Drobo itself will indicate the percentage of disk being used, if you are running low on disk capacity, or if a drive has failed.

Like all mechanical and electronic devices, a Drobo can fail. Drive failure protection, such as that provided by Drobo or a RAID array, should never be considered a replacement for proper backup procedures. Drobo provides a very good guide to data protection here:

http://www.drobo.com/support/best_practices.php

The Drobo B800i

The Drobo B800i is an eight-slot iSCSI SAN with two Gigabit Ethernet iSCSI ports and one USB 2.0 port. The USB port is used primarily for initial setup, although technically it can be used as the data connection as well, albeit with greatly reduced performance. Drobo describes the B800i as a “cost-effective iSCSI SAN for businesses needing primary or backup storage.”

The B800i’s iSCSI connections can be used in a number of configurations. Here are some common approaches:

A single host connected directly to an iSCSI port;
A single host connected directly to both iSCSI ports when the unit is configured in Network Failure Protection mode;
Two hosts connected directly, one to each iSCSI port when the unit is not configured in Network Failure Protection mode;
A single or multiple hosts connected to a network switch dedicated to iSCSI, with a single iSCSI port or both ports when the unit is configured in Network Failure Protection mode;
Single or multiple hosts connected to two dedicated switches, each connected to a single iSCSI port when the unit is not configured in Network Failure Protection mode;
A single or multiple hosts connected to a non-dedicated switch, with a single iSCSI port or both ports when the unit is configured in Network Failure Protection mode.

Direct connections provide the best performance. If connecting to multiple physical hosts or iSCSI SANs is needed then using a switch and network dedication to iSCSI traffic is recommended. Connecting to a network and switch shared with other network traffic is not recommended because it can impact both iSCSI performance and the performance of other network traffic as well.

Drobo Dashboard allows you to initially configure the array using the array’s USB port. After initial configuration you can power down the Drobo, disconnect from USB, and then connect to one or both iSCSI ports. Once the Drobo is reconnected and powered up Drobo Dashboard will discover it. Dashboard can now be used to manage and monitor the array.

VM Support

Drobo officially supports VMware and XenServer virtualization hypervisors on the B800i, but not all versions are supported. Currently there is no official support for Microsoft’s Hyper-V, but Drobo has reported that some customers have it working.

SSG-NOW Assessment

In our limited testing the B800i worked as advertised. It survived the removal of a drive in single-drive protection mode and survived the removal of two drives in dual-drive protection mode. When drives fail or are removed LEDs on the Drobo, as well as the display on Drobo Dashboard, indicate that a failure has occurred and that a rebuild is in progress. The rebuild time is usually several hours, but the system still functions properly during the rebuild process, albeit with reduced performance. In testing, the performance impact during rebuild was approximately in the 40% range under heavy I/O load. Under normal loads the impact should be fairly negligible. During the rebuild Drobo Dashboard displays the amount of time the rebuild is expected to take, but we did not find this to reflect the actual rebuild time. In some cases rebuild took less time, in others more.

A few improvements, SSG-NOW would like to see in the B800i:

Port aggregation, to allow both iSCSI ports to connect in parallel to improve performance.
Better support for Linux, with something equivalent to Drobo Dashboard.
Support for Microsoft’s Hyper-V.

With a list price of $3,499 without drives the B800i isn’t cheap, but pricing is in line with other iSCSI SANs of similar capacity. What the B800i provides that competitors don’t is an amazing amount of flexibility. Drobo allows you to add and remove drives, change disk capacity and change the protection level pretty much at will without affecting operation. Thin provisioning allows you to create volumes larger than existing disk space and add capacity as needed. Other than initial volume configuration nearly all functions can be handled from the front of the Drobo itself. LEDs tell you if capacity is low or if a drive has failed (and which one) and if a rebuild is in progress. If you have enough disk space Drobo will rebuild itself using the existing drives and still provide the protection level you have chosen. If not, Drobo will continue operating without protection until the failed drive is replaced.

Replacing a failed drive is simplicity itself–press the lever below the drive (a red LED indicates which drive has failed) to eject the drive then hold the lever down to insert a new drive. You don’t have another drive of the same capacity? No problem–you can add a larger drive or, if you have enough free disk space, even a smaller drive. Since Drobo supports all current SATA drives, a B800i can currently be expanded to 24TB using eight 3TB drives. As larger drives become available Drobo should be able to handle those as well.

Overall, the B800i is an easy-to-use, easy-to-install and easy-to-manage SAN with exceptional flexibility and expansion potential.