Managing data storage is just as much of a task (or greater) as managing the servers themselves. It makes sense to centralise management in larger organisations wherever possible. Storage area network (SAN) technology enables a network of physical storage devices to be managed as a single unit. They can then be readily divided into a number of logical drives.
Basic SANs use either Fibre Channel (FC) or iSCSI technology. iSCSI runs over standard gigabit Ethernet networks using software to support the SAN infrastructure, whereas FC devices are much faster but require specialised hardware such as optical fibre cabling and FC switches.
SAN vs. NAS
Storage area networks and network attached storage (NAS) are similar names and easily confused. The key differences are:
- The manner in which stored information is presented to servers. Data stored in a NAS will be seen on a non-local drive whereas data stored on a SAN appears to reside in a local drive.
- Physical hardware. A NAS is a single storage device, whereas a SAN is a centralised collection of networked storage devices. Multiple servers could require an individual NAS each, but a SAN allows sharing of a single storage area.
- Administration. SAN space can be allocated as needed to each server, often on-the-fly. Administrators can determine a total amount of physical drive space required without concerning themselves with how to physically allocate space to each server.
- A NAS is optimised for sharing while a SAN is optimised for speed and capacity.
Comparison of NAS and SAN (Credit: Enex TestLab/CBS Interactive)
NAS/SAN hybrids are also possible where one or more partitions in the SAN are provided with a NAS gateway.
Storage clustering is a method of combining networked storage devices to form a single virtual device. It allows the creation of large, redundant storage arrays which remain relatively easy to manage. Since clustered systems contain multiple disks, each with its own read-write head, performance can potentially be multiplied by simultaneously accessing multiple physical devices.
Available in a variety of flavours, Redundant Array of Independent (or Inexpensive) Disks (RAID) is a method of combining several physical drives into a single logical unit with varying levels of data redundancy. The most commonly seen varieties are 0, 1, 5, 6, 10, 50 and 60. RAID 0 (also called striping) connects several identically sized volumes together, increasing read and write speed but offering no redundancy. RAID 1 (also called mirroring) is a 1:1 copy of a disk, so if one disk fails, there is a redundant copy — it should increase read speeds, but has a penalty on writes and can be costly — for every disk you wish to protect, you must purchase an additional identical capacity disk.
RAID 5 and 6 are the compromise — both allow a collection of disks of identical capacity to be connected together to form a single logical unit, but rather than the expense of having to purchase a drive per drive you wish to protect, they spread the parity data across all the disks in the array. RAID 5 can survive a single drive failure, RAID 6 can survive two.
RAID 10, 50 and 60 are combinations of RAID 0 plus RAID 1, 5 or 6. Some SAN vendors support non-standard RAID varieties or less common standards such as RAID 3 or 4.
Consider an organisation where data storage is shared equally between two servers. If one server is discovered to use its virtual disk space faster than the other, it may be necessary to purchase more drives to satisfy the capacity needs of this space-hungry server even though there is still storage space available on the other server. Thin provisioning is a method of allocating available space to each virtual drive as needed. This has the potential to save organisations from purchasing vast amounts of hardware that may never be fully utilised.
The PowerVault MD3000i is Dell's entry-level SAN storage solution. The standard version includes one controller with 512MB cache. A high availability version is also available with dual controllers providing active/active redundant systems.
The MD3000i supports RAID 0, 1, 5 and 10. This seems a little poor given the broad use of RAID 6. There could be as many as 15 disks in this device. It really needs an enhanced RAID controller to provide better data safety. Expansion modules can push the total number of drives even further, 45 drives with a total capacity of up to 18TB per enclosure (and a grand total of 90TB with additional enclosures).
With no option for fibre channel the array has to share the gigabit TCP/IP network port to transfer data. 1Gbps Ethernet NICs are likely bottlenecks for this array. If configured with faster SAS disks rather than SATA, the limitation would be particularly obvious. Each installed controller has 2 gigabit ports.
Dell provides a number of optional management software features including snapshots and disk copy; however, these are not packaged with this unit (they really should be included in any default configuration). Thin provisioning is another feature that will be missed in an entry-level SAN such as this.
Hot-pluggable technology can enable administrators to easily upgrade disks while keeping the business running. Though in truth, hot-pluggable disks are probably of greater use in a repair scenario rather than when upgrading overall capacity. This can be done during occasional scheduled maintenance. A maximum of only 16 servers can be connected to the MD3000i.
This unit also supports 5400rpm SATA disks. Dell claims the new disks will decrease power consumption by up to 30 per cent. This is definitely good news for MD3000i, because its maximum power consumption is rated at 550W — the highest of all SAN models reviewed.
Windows and Linux operating systems support is claimed. Dell uses a web-based management console, which bring flexibility to administrators.
- RAID controller-based iSCSI SAN solution with much better performance than PC-based ones
- Supports both SAS/SATA disks
- Two controllers with mirrored cache provide active/active redundant
- Doesn't support RAID 6
- Only supports Windows and Linux operating systems
- Snapshot and disk copy are extra costs
The bottom line
Up to 15 disks and 16 server connections make this ideal for small enterprise and non-key applications.
NetApp is a young but dedicated player in the storage field. NetApp provides a range of handy management proprietary technologies in its FAS2020. These include: RAID DP (based on RAID 6) for data stability and FlexShare for workload prioritisation and FlexVol. FlexVol is a useful technology for a business with increasing demands. Administrators don't need to know in advance how large individual volumes need to be for a given application; volumes expand as demand grows (so long as total SAN capacity is not exceeded). Aside from the basic management package, additional software packages are available to suit more specialised or high-end business needs.
The FAS2020 can have up to 1024 LUNs and 51,000 snapshots. This clearly exceeds the demands of mid-sized enterprises. However, it proves that the FAS2020 can accompany your business' long-term growth. The maximum disk capacity for the FAS2000 series devices is 68TB.
Active/active redundancy with failover controllers is an added safety feature for critical scenarios. It means you can hot-swap failed controllers without a halt to business. Both Fibre Channel and iSCSI protocols are supported, allowing buyers the choice of higher speed or lower costs.
NetApp FAS2020 provides plenty of valuable features (although some come at a cost) such as SecureAdmin, thin provisioning, host scripting, email alerts, virus protection and role-based access control.
NetApp claims (based on a report by Oliver Wyman) that its product has a very low total cost of ownership when compared to competing products, mainly due to more efficient use of disk space. Energy usage also appears to be low.
The FAS2020 supports a wide range of operating systems including Windows, Linux, Solaris, AIX, HP-UX, Mac OS and VMware.
- RAID DP greatly improves data safety
- Large snapshot capacity
- Up to 1024 LUNs
- FlexVol is useful for a growing business
- Direct Snapshot support for key applications (Exchange, MS-SQL, Oracle, SAP, VMware etc)
- Connected host number is limited to 24
The bottom line
A very good product and useful software, but limited baseline warranty/support service.
Who says a server can't be classy? The AX4 has a fine looking facade styled with attractive lines and shades of silver and grey. Opening its access panel reveals 12 2.5-inch 750GB SATA hard drives (minimum four, and part of this is reserved for housekeeping). Dell's AX4-5f is an OEM version of EMC's Clariion AX4. The specifications from EMC indicate a DC power source whereas the information from Dell indicates AC power; in fact, the EMC tested at Enex (which was provided directly from EMC) was endowed with both options!
Dual controllers are now the standard for entry-level SANs and the AX4 is no exception. It provides good flexibility in both active/active and active/spare scenarios.
A single AX4 can provide storage space for up to 10 servers. With the help of expansion packs, the number can be increased to 64 servers (and up to 60 drives). This is a reasonable number for any mid-sized enterprise and they can connect all their servers on one array and establish data storage in a really useful manner. Storage capacity for the AX4 is 27TB with current SAS drives and 60TB with SATA II drives.
Support for both 1Gbps iSCSI and 4Gbps Fibre Channel provides choices for varied budgets. Users can select high performance or low cost. Similarly, support for both SAS and SATA disks provides a performance versus cost trade off. In fact, one can mix both types of drives in one SAN system to suit individual company needs.
RAID 5 is a bit old hat these days. With the dramatic increase in disk numbers per array, administrators need more stability to keep data safe. More disks mean more risk of disk failure. RAID 6, as provided by both devices, is really essential for disk storage systems because it doubles parity checking protection. The AX4 ensures companies have this added security.
The default Navisphere management software only supports 16 snapshots, which is obviously not enough for any productive environment. However, the EMC SnapView software pack can be purchased to enable administrators to create as many as point-in-time snapshots as space allows.
The AX4 has enhanced integration with VMware solutions. This means users can easily build or migrate to a virtual environment. The AX4 also allows for thin provisioning of volumes which reduces the administrator's need to be adept with a crystal ball.
EMC's DC power option greatly improves the capacity to use the AX4-5 in the telecommunication field. A wide range of OS support is provided including Windows, Linux, AIX, HP UX, Netware, Solaris and VMware.
The standard warranty is for three years with 24/7 remote support. A premium support service is available which includes on-site support with a four-hour response time.
- Integration with VMware
- UPS-backed mirrored cache and cache-destaging to disk
- Standard snapshot support is inadequate
The bottom line
The AX4 makes it a safe choice as it has all the essential SAN features.
The HP LeftHand Starter pack consists of a pair of 2RU devices built for function rather than beauty — as would be expected for a server-room machine. As provided to Enex, each unit holds eight 300GB SAS drives plus a DVD ROM drive. Thus the total capacity is 4.8TB. A 12TB SATA version of this SAN is also available. The number of drives is expandable from 16 to 256 with the addition of extra nodes. Each unit has dual active/active controllers.
Clustering of storage units provides for a straightforward expansion of the SAN. Data is striped and mirrored across all volumes in the cluster, preventing a failure in a node resulting in data loss. Thus larger organisations can benefit from very impressive data stability. When adding extra storage units to the SAN the system can remain online, reducing maintenance downtime. Thin provisioning eliminates the need to predict the amount of space each volume may require in the future. Additionally, administrators can choose to set redundancy on a per-volume basis (up to four times).
Given the capability and scalability of this product the user interface is convenient to use. Basic configuration of the SAN, including IP address, is initially configured under a simple text interface on the SAN itself — which allows for direct keyboard and monitor connection. The majority of the device set-up is then configured via a GUI installed on a server.
Inside the box, everything is well laid out for easy access during maintenance. Hot-swappable parts such as fans, drives and power supplies are easy to remove.
The HP appears to support just about every OS known to man, including Windows, Linux, Citrix, OS Snow Leopard, IBM AIX, HP-UX, VMware, Novell and Solaris.
Due to its size (at least in part) this set-up is quite power hungry — 293W per unit. HP provides a three-year on-site warranty with next day service and nine-to-five software support.
- Very good expandability in terms of ease and scale
- Excellent OS support
- Thin provisioning
- Standard support contact includes office hours only
The bottom line
If there was FC support, HP LeftHand P4300 would be fabulous.
This device is a work horse and is unapologetically focused on engineering rather than appearances.
IBM has always been a competitive player in storage field. The DS3200 is quite similar to the DS3400, except that the latter also supports fibre channel in addition to iSCSI. The DS3400 comes in three flavours: single controller, dual-controller and dual-controller telco (with 48V DC input).
IBM claims the DS3200/DS3400 is appropriate for mid-sized enterprise because of its expandability. The DS3200/DS3400 can accommodate up to 48 disks — although this figure has been exceeded by some other products on the market (the EMC AX4, for example, can be expanded to take up to 60 drives). The base DS3200/DS3400 unit can support up to 3.6TB; with 14.4TB capacity being possible with the addition of three EXP3000 devices.
The DS3200/DS3400 can be upgraded to two controllers with 2GB cache, and six 1Gbps iSCSI ports (DS3200) or four 4Gbps FC ports (DS3400).
A useful feature of the DS3200/DS3400 is DC power support compliant with NEBS-3 and ETSI. This means it could even be used in a telco datacentre. Some mid-sized enterprises rent space in an independent datacentre instead of building their own. The power options of this device really broaden its application and location options.
The management software for this unit seeks to be task oriented rather than object oriented. Thus when an administrator seeks to make a change the first question is what process rather than what device is to be operated on. The reasoning for this is that it makes it easier for non-experts to grasp — which is not an unreasonable premise. Concepts such as copying and expanding are, for example, simpler than appreciating the difference between virtual and physical storage devices. Even so, if usability is a concern, IBM might have also thought about including thin provisioning.
The DS3200/DS3400 consumes a maximum of 361W. This is lower than most of the other devices considered; however, it is difficult to assess how the wattage per operations performed compares between this machine and the others reviewed here.
OS support is very good including Windows, Linux, Netware, AIX, VIOS, Solaris and VMware.
The DS3400 comes with a three-year parts-and-labour warranty.
- 48V DC power support
- Good interoperability with IBM Tivoli storage management solution
- IBM's claimed support for SAN switch models is limited
- Disk expansion is limited to 48
- Lack of active/spare mode in the RAID controller
The bottom line
Six iSCSI ports and DC power support are the only two shining points of this device which otherwise does not excite.
Sun's product is very similar to the IBM DS3400 both in terms of features and physical form. Like IBM, Sun produces well engineered products. It is a 2RU unit with space for 12 SAS or SATA drives. The ST2510 supports only iSCSI while the ST2540 can work both iSCSI and Fibre Channel. The Sun machines have up to eight 1Gbps iSCSI ports (ST2510) or four 4Gbps FC ports (ST2540), and up to 48 disk capacity with expansion units allowing for up to 48TB of storage. DC power support makes it IDC-ready.
The eight iSCSI ports could reduce the need for gigabit Ethernet switches; however, up to 1GB cache might limit their performance in FC mode.
Thin provisioning is not included in this device — Sun provides this service with high-end devices, whereas the 2510/2540 family are entry-level SANs.
The StorageTek 2510/2540 is also snapshot ready (sadly, this function is an optional extra). With up to 128 shots for each array, it may not be enough for some critical scenarios.
There can be as many as 30 hot spares in an ST2540. While this is impressive, few organisations may need so many spares at one time.
The management software is web-based for easy access to tools including port-to-volume mapping, volume creation capacity expansion and diagnostics.
This Sun device supports a good range of operating systems including, Windows Server, Solaris, SUSE and Red Hat as well as VMware.
The StorageTeks standard warranty is three years, though the device must be returned to base in the event of a warranty claim and phone service is during office hours only. Naturally, higher levels of service are available as an option — including 24/7 phone support and rapid, on-site servicing.
- AC/DC support
- Up to 30 hot spares
- Only 256 LUNs
- Lacks support on IBM AIX and HP-UX
- Power consumption is relatively high (515W max)
The bottom line
Eight iSCSI ports and DC power support are the only two shining points of this device which otherwise does not excite.