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3ware Hardware RAID vs. Linux Software RAID

As part of a project we are building a iSCSI storage server. It has 16 500GB SATA disks and to provide redundancy we needed some sort of RAID controller. So we went with the well known 3ware controllers. More specifically a 9550SXU 16ML. The server itself is running CentOS 5 with 2 Dual-Core Intel Xeon processors. This post is just to share my experiences with this controller.

First, everybody that uses a ext3 filesystem on top of this controller should upgrade to firmware 9.4.2. It improves the write performance. In our case we went from around 55MB/s to around 75 MB/s for sequential I/O. That is not bad for a simple firmware upgrade.

But we were having one issue. On initial tests with a iSCSI exported logical volume we did a copy of a directory tree (on the same volume) with a total size of 1GB. And this test took around 6 minutes, that is even less then 3MB/s. To be fair, this structure contains a lot of smaller files, different directories. So a copy also involves a lot of metadata activity. So we did the same test on the storage server itself and we got around 3 minutes there. Mmm, and this should be a top end RAID controller, but we are only getting 6MB/s in this test.

So, just for fun I decided to let the RAID controller export each disk individually and use the Linux software RAID and see what performance that would give. Well, for this specific test the time was 2’50”. And for all the other tests I did software RAID outperformed the hardware RAID.

So what did we learn here. First, do not let the numbers of all the different hardware RAID controller vendors fool you. They are sequential I/O test. But most day-to-day I/O patterns are different.
Second, always test for yourself. Benchmarks found online can be indicators, but always test everything yourself for your specific case.

Third, when using RAID always give software RAID a chance. It may save you some money, but if performance/reliability is what you are looking for then let a back end system handle it.

As a final note. If we mounted the filesystems as ext2 in this specific test the copy would only take 1 minute (in both HW and SW RAID). So do not forget about ext2, it still has it’s advantages for performance.

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