Adding storage to a server using a RAID adapter

Logan Harbaugh tests RAID adapters from Adaptec and LSI with Fujitsu and Seagate SAS drives and a Western Digital RAID-class SATA drive and details how to add storage to a server using a RAID adapter.

Servers have a way of growing, from basic to advanced, as users add more and more files, and administrators add more functionality. A basic server that's really only a desk-side PC may be running out of storage, and rather than buying a new server with internal RAID storage, or adding an expensive Fibre Channel controller and external storage, it's entirely possible to add a RAID controller and either SATA or SAS disks internally, for relatively little money and with most of the advantages of an expensive new storage server.

Say you have a server, a reasonably recent system with enough memory, processing power and reliability, but you've discovered that some of the applications running on that server are really crucial -- business will stop if they're not available, or demand for storage space is greatly exceeding the demand. You really don't want to have to buy a whole new server, move the operating system and applications to the new box, etc. You just want to add some reliable storage.

There are several ways to add storage to an existing server. One option is to add a RAID adapter and three disks to 12 disks internally, assuming you have enough space and power. Adding a single drive to the existing SATA connector on the motherboard of your server is very inexpensive, and is easy to do, but one disk isn't very fault tolerant -- if there's a hardware problem, all your data could disappear.

RAID adapters come in two basic flavors -- SAS/SATA and SCSI. SCSI RAID controllers have been around for many years, and are supported natively by most current operating systems, which means you won't have to install drivers to get them to work. They support both internal and external drives, so you can add drives to an external enclosure if you don't have the space inside your server. Generally, SCSI drives and a SCSI RAID controller will yield better performance than a SAS/SATA controller and SATA drives. On the downside, SCSI adapters are relatively expensive, and require SCSI drives, which are much more expensive than SATA drives -- as much as 10 times more expensive in cost per megabyte.

SATA-only RAID controllers have been around for several years, and have been through several generations of improvements. The latest is that controllers from the major manufacturers, such as Adaptec Inc., Atto Technology Inc., LSI Corp. and Promise Technology Inc. support both SATA drives and SAS drives, using the same interface, cables and so forth. This means that you can add inexpensive SATA drives, and if you need more performance, you can add SAS drives without having to start over with the rest of the system. Some controllers also support externally-connected drives using an eSATA connector.

Drives will be a major part of the cost of a storage upgrade, and there are a couple of things you should know about them. There are really two types of drives, SATA and enterprise. Enterprise drives include SCSI, Fibre Channel and SAS -- even though the interfaces are different, the internal components are the same, and are typically built for very high reliability and performance. They are also much more expensive than SATA drives. SATA drives come in two flavors: standard drives, and enterprise class or RAID class (depending on the manufacturer). RAID-class drives are built to stand up to the 24/7 operation in server RAID applications.

So, if you decide to add a SAS/SATA controller to an existing server, what can you expect? Most of the controllers available support eight, 12 or 16 drives, using standard four-into-one fan-out cables. As long as your server chassis has enough space, and a big enough power supply, you can add up to 16 TB. Performance will be primarily dependent on drives and the type of RAID you use. SAS drives will provide better performance than SATA drives, but at considerable extra cost per megabyte. RAID 5 is the most efficient in terms of getting the most usable storage, but RAID 0 or 1 (or a combination known as RAID 0+1 or RAID 10) will give you higher performance.

Installing the adapters is straightforward, requiring either a PCI or PCI Express slot, depending on the model. If you get the PCI Express model, pay attention to the slot requirements -- many servers have a 1x/4x slot, and some cards, such as the LSI 8888ELP, require an 8x slot. If you have a case with tight clearances around the motherboard, you should also be aware that the cables need three to four inches beyond the end of the adapter. Once the adapter is installed and the drives installed and connected, power the system back on.

During the boot process, you'll have the option to configure the card, to set up the type of RAID you'll be using and to initialize the disks, build the array, and configure some options such as read/write caching. Once that's done, the system will continue booting, and the operating system should recognize the new hardware and prompt you to install the drivers. You will notice a delay while the controller initializes, which may be from 15 seconds to 45 seconds.

I looked at two adapters, the Adaptec 3805, and the LSI 8888ELP. Both support up to eight drives, and offer performance comparable to classic SCSI adapters using SAS drives, with the option to use the much less-expensive SATA drives as well. I tested both adapters with three sets of drives, eight each of a Fujitsu 36 GB 15k RPM 3.5-inch SAS drive, a Seagate Savvio 73 GB 15K RPM 2.5-inch SAS drive and a Western Digital RAID-class 500 GB 7200 RPM 3.5-inch SATA drive. Test results are show in the tables below.

The biggest differentiator between the two adapters is that the Adaptec was able generate performance in MBps with SATA drives comparable to SAS drives, while the LSI had excellent SAS performance, but showed a drop-off in MBps with SATA drives. On the other hand, the LSI achieved a higher number of I/O operations per second with SAS drives, by quite a margin: 77241 IOps vs. 56291 IOps for the Adaptec.

Performance in IOps will make a difference if you're running a database, or some other application where users are indexing or looking through large numbers of files. For basic file server performance, MBps will give you a better idea of performance, and over 200 MBps means that the server will be able to keep up with a gigabit Ethernet connection and not create a bottleneck.

Either of these adapters will allow you to add up to eight drives (up to 16 with other models) to a server, with easy installs and much better performance and reliability than you'll get using the adapters built into the motherboard.

Adaptec test results


Drive Random access in milliseconds % CPU utilization Average read in MBps
36 GB SAS 6.1 4.3 257.7
73 GB SAS 5.9 5.2 256.7
500 GB SATA 11.7 4 230.4

LSI Logic test results


Drive Random access in milliseconds % CPU utilization Average read in MBps
36 GB SAS 5.75 3.8 262.7
73 GB SAS 5.55 2.5 175.1
500 GB SATA 11.5 1 65.1

Disk drives used

Fujitsu MAX3036RC SAS-D, 36 GB 3.5-in SAS, 15k rpm, 16MB cache, five year warranty. Bulk price $135-145 ea.

Seagate ST973451SS, 73 GB 2.5-in SAS, 15k rpm, 16MB cache, five year warranty. Bulk price $365-500 ea.

Western Digital WD5000ABYS, 500 GB 3.5-in SATA, 7.2krpm, 16 MB cache, five year warranty. Bulk price $125 ea.

IOMeter results with 36GB SAS drives


Adapter Average IOps Max IOps Average MBps Max MBps
Adaptec 3805 1408.79 56291.1 5.5 614.23
LSI 8888ELP 825.56 77241.2 3.22 606.70

Logan G. Harbaugh is a freelance reviewer and IT consultant in Redding, CA, who has worked in IT for more than 20 years.


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