On Thursday February 20, joker@astonia.com wrote: > Hi again, > > I've received some helpful responses, and I'd like to share those, and the > new test results. Let me know if I'm boring you to death. ;) > > One suggestion to speed up the reads was to issue several reads in > parallel. Silly me didn't think of that, I was completely focused on the > writes, which are more important for my application. Anyway. Using parallel > reads (from four processes), read performance scales almost with the number > of disks in the array. This goes for both, hardware and software RAID, with > software RAID being about 15% faster. > > Write performance on the other hand does not change at all when using > multiple processes - for obvious reasons: The kernel queues, sorts and > merges write requests anyway, so the number of processes doing the writes > does not matter. But I've noticed something peculiar: If I change my > benchmark to write 4K blocks at 4K boundaries, write performance increases > to almost 300%. This is quite logical, since the kernel can write a 'page > aligned' block directly to the disk, without having to read the remaining > parts of the page from disk first. The strange thing is that the expected > performance gain from using RAID0 does show when writing aligned 4K blocks, > but not when writing unaligned blocks. Non-aligned writes also tend to > block much more often than aligned writes do. It seems the kernel doesn't > handle unaligned writes very well. I can't be sure without having read the > kernel sources (which I don't intend to do, they give me a headache), but I > think the kernel serializes the reads needed to do the writes, thus killing > any performance gain from using RAID arrays. When you do unaligned writes to a block device, it pre-reads the parts of each page that you don't write. This causes your loss of perfomance. I *think* (i.e. vague memory from reading code suggests) that if you open with O_DIRECT and make sure all your accesses are 512 byte aligned and a multiple of 512 bytes in size, it should avoid the pre-reading and should give you full performance. NeilBrown - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
