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Are RAID Controllers the Next Data Center Bottleneck?
storagedude writes "This article suggests that most RAID controllers are completely unprepared for solid state drives and parallel file systems, all but guaranteeing another I/O bottleneck in data centers and another round of fixes and upgrades. What's more, some unnamed RAID vendors don't seem to even want to hear about the problem. Quoting: 'Common wisdom has held until now that I/O is random. This may have been true for many applications and file system allocation methodologies in the recent past, but with new file system allocation methods, pNFS and most importantly SSDs, the world as we know it is changing fast. RAID storage vendors who say that IOPS are all that matters for their controllers will be wrong within the next 18 months, if they aren't already.'"
There may need to be some minor rethinking of controller throughput for read applications on smaller data sets for SSD. But right now, I regularly saturate the controller or bus when running sequential RW tests against a large number of physical drives in a RAID{1}0 array, so it's not like that's anything new. Using SSD just makes it more likely that will happen even on random workloads.
There are two major problems with this analysis though. The first is that it presumes SSD will be large enough for the sorts of workloads people with RAID controllers encounter. While there are certainly people using such controllers to accelerate small data sets, you'll find just as many people who are using RAID to handle large amounts of data. Right now, if you've got terabytes of stuff, it's just not practical to use SSD yet. For example, I do database work for living, and the only place we're using SSD right now is for holding indexes. None of the data can fit, and the data growth volume is such that I don't even expect SSDs to ever catch up--hard drives are just keeping up with the pace of data growth.
The second problem is that SSDs rely on volatile write caches in order to achieve their stated write performance, which is just plain not acceptable for enterprise applications where honoring fsync is important, like all database ones. You end up with disk corruption if there's a crash, and as you can see in that article once everything was switched to only relying on non-volatile cache the performance of the SSD wasn't that much better than the RAID 10 system under test. The write IOPS claims of Intel's SSD products are garbage if you care about honoring write guarantees, which means it's not that hard to keep with them after all on the write side in a serious application.