“Aloha Open Systems Storage Guy,
I’m a recent convert to storage administration. I’m having a hard time cutting through the cruft to find the truth. Could you answer some of these questions?
1 – Which is faster, software-based RAID (e.g. Linux md, Windows Dynamic Disks) or hardware-based RAID? One person said that software-based RAID is faster because it has a faster processor and more RAM/cache (something like a Xeon 3.0 Ghz w/ 4Gb or RAM would be typical in my environment). But how could that stack up against my (little bit old) IBM DS4300 Turbo (2Gb cache).
2 – Which is faster, RAID-5 or RAID-10 (or is that RAID-01?) I know everybody says RAID-10, but what about those fancy XOR engines? Or have I fallen prey to marketing?
Thanks for taking a moment to listen to my questions.
Mahalo (Thank you),
Hi John, and welcome to the blog!
To answer your questions, I’m first going to give a bit of background info. If any of my statements don’t make sense, please reply and I’ll answer :).
The term “faster” can mean different things to different people. Each type of storage has its strengths and weaknesses, and different applications perform differently on the same storage systems. There are two primary application workloads- those that do random IOs, and those that do sequential IOs.
The random workloads are the hardest ones to provide storage for because it’s very difficult to “read ahead” by predicting where the next read will fall. An example of an application that has a random workload would be a database or email server.
The sequential workloads are easier to provide storage for. Pre-fetching the next block will most of the time yield a read that’s already in cache. An example of an application like this would be a backup server or certain file servers.
Another general bit of info is that in a RAID, reads (not writes) are usually the bottleneck. Writes are usually fed into the cache and acknowledged to the host server immediately. Reads, however, are typically 70% of the IO being done by a system, and as we discussed are often impossible to “pre-cache”.
When you’re calculating performance, the two stats you’ll want to know is IOs per second for random loads, and MB per second for sequential loads (abbreviated IOPS and MBPS). When you’re trying to tune a system to be quick for your applications, you need to know the different levels of your system and which one is the bottleneck. Normally, on a decent controller, the number of spindles you have in the RAID will determine the IOPS. You should get a linear increase in performance as you add drives to a RAID. Cache is important for the 30% of writes you can expect (your mileage may vary), however everything goes to disk eventually, and most people experiencing slow performance on their disk controllers simply don’t have enough disks.
Onto the specifics of your question:
1- Software or Hardware RAID: For most workloads, a dedicated hardware RAID controller is faster. Software RAIDs have to share resources with the operating system, which is usually not optimized for sharing on that level. The IBM DS4300 you have is actually an LSI box, and has a very powerful RAID controller for its price. Don’t let your sales rep try to replace your controller! Those boxes may be a little old, but the only major difference between that and the newer IBMs is that the newer ones use 4 gig fiber and more cache. It’s very rare that a workload can max out 2 gig fiber on the front end, and even more rare that the controller can fully utilize all the bandwidth on the disk side. The extra cache can be useful, but you will experience diminishing returns- the benefit of going from 2 to 4 GB is way less than from 1 to 2 GB. The controller should not be your bottleneck for anything under 80 FC drives on the system you have, so unless you want to go beyond that, keep your box until the maintenance costs more than the replacement. Add more drives if you need IOPS or MBPS, but don’t throw it out. These boxes are supposed to be like houses- only buy a bigger one when you need it. Not because the last one is obsolete.
2- RAID 5 or RAID 10: I will compare them in reliability and performance. RAID 5 uses the space of one disk for parity, and RAID 10 uses the space of half the disks. Reliability wise, RAID 10 is the obvious winner. You can lose up to half your disks before you lose data (assuming you don’t lose two of the same pair). If you lose a second drive while rebuilding a critical RAID 5 array, you will always have to go back to your last backup. Generally, this is more of a worry for large SATA drives than it is for the smaller and faster FC drives- SATA RAIDs take exponentially longer to rebuild because of the larger amount of data combined with the lower performance per spindle.
Speaking of performance, the performance (per drive) is better on RAID 5. Most people put two RAID 5s on each enclosure, and have 4 to 6 RAIDs per hot spare. The XOR engine you speak of performs the parity calculations for RAID 5, however is not needed for RAID 10 or any other non-parity type of RAID. Since you do have a fairly fast controller, RAID 5 is attractive, however you have to balance your decision based on performance and reliability.