Electronic Design
Hands-On A Hybrid SAS RAID Adapter

Hands-On A Hybrid SAS RAID Adapter

I have been working with the Adaptec RAID 6805 controller (see Low Cost SAS RAID Controller Handles SDD/HDD Mix) that is part of the low cost, entry-level Series 6 from PMC Sierra. I have been working with it for a couple months now and have been very impressed with its price/performance and flexibility. The eight port 6805 (Fig. 1) has a x8 PCI Express 2.0 interface with bandwidth of 1600 Mbytes/s. The 6405E is the four-port version with a x1 PCI Express 2.0 interface with bandwidth of 400 Mbytes/s. The Series 6 uses the PMC-Sierra's PM8013 SRC 6Gbit/s RAID-on-Chip.

The Series 6 brings a number of features to the entry level environment including a RAM cache that can be backed up using a supercap and the hybrid RAID support we will be looking at here. The 128 Mbytes of DDR2-800 DRAM of cache provides a significant performance boost. This approach is is quite common on higher end SAS RAID controllers.

It support SAS and SATA solid state disks (SSD) and hard disk drives (HDD). The boards support RAID 0, 1, 1E, 10 and JBOD. They also provide hybrid RAID support that combines the best of SSDs and HDDs.

The Disk Drives

I used the system with a pair of 100 Gbyte, 2.5-in Micron P300 RealSSD SATA SLC SSDs and 3 Tbyte, 3.5-in Seagate Barracuda XT SATA HDDs. Both provide 6 Gbit/s transfer rates. The Barracuda XT has a 64 Mbyte cache. The 7200 rpm drive provides a good mix between capacity and performance. It targets consumer applications. I have some Seagate enterprise Constellation ES drives that would provide a more robust solution but they were in use already.

The Micron P300 RealSSD drive is designed for enterprise applications. It has steady state sequential read/write performance of 360/255 Mbytes/s. Its random read/write performance is 44,000/16,000 IOPS. The drive supports secure erase and has static and dynamic wear leveling. The endurance of the 100 Gbyte drive is 1.75 Pbytes. The drive weighs in at only 100g.

I checked out the system using the drives in a number of combinations (Fig. 2). Virtual drives are configured using the RAID controller's BIOS interface. The standard configuration is pairing the SSDs and the HDDs in a RAID 1 configuration. Any SAS controller could handle this configuration. The performance of the Micron P300's (Fig. 3) is great but I was able to get the same kind of performance using the hybrid RAID configuration.

For the hybrid configuration, I paired an SSD and HDD to create a 100 Gbyte virtual drive. This resulted in two 100 Gbyte virtual drives. The remaining space on the HDDs (3 Tbyte less 100 Gbytes) was a third drive. This approach provided three virtual drives. Two were essentially as fast as the RealSSD drive and one was slower but much larger. In hybrid mode, data is written to both drives but it is read from the faster, flash drive. RAID 10 could have been used to provide a fast 200 Gbyte RAID configuration.

Configuring the drives was the easy part. The Series 6 is so new most operating systems do not have the drivers bundled with it. This means installing the drivers from CD. I tested the system using CENTOS 6. This is a free version of Red Hat Enterprise Linux 6 (RHEL6). I did run into an issue because the standard configuration was not recognized so I had to run the pre and post install scripts manually but the system worked just fine once the drivers were installed.

I also installed the multiplatform, Java-based, Adaptec Storage Manager (ASM). This management tool works with the device driver and a monitor daemon to handle the controller when the operating system is up and running. I have three Adaptec controllers spread across various servers in the lab and was able to manage them all from a single console. It is very handy when a drive fails and the alarm on one of the servers goes off.

One of the reason's the Series 6 gets such good performance is the on-board RAM. It is definitely worth the money to invest in the Zero Maintenance Cache Protecgtion super-cap. This keeps the RAM alive when the system fails. It charges quickly unlike a backup battery.

Adaptec' 6805 definitely ups the ante for low to mid-range servers. It was easy to setup and delivered the kind of performance I expected. Check it out if you need performance on a budget.

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