On Thu, Feb 17, 2011 at 10:52:07AM -0800, Patrick J. LoPresti wrote: > I have a fair amount of experience with hardware RAID devices, but now > I am investigating Linux software RAID and I have a question. Well, a > few questions. > > The classic problem for RAID5/RAID6 write performance, especially when > striping across many drives, is that a single small write requires > reading in the entire stripe from all disks to calculate the new > syndrome block(s). > > Hardware RAID controllers typically mitigate this problem by using a > sizable (512MiB - 4GiB) non-volatile write-back cache, in the hopes > that enough blocks will be written in a short period of time to > populate an entire stripe. Once an entire stripe is in the write-back > cache, it can be written out with its syndrome blocks without having > to read anything. > > Of course, the cache has to be non-volatile (battery backed or solid > state), because the kernel is expecting stuff it has written to disk > not to vanish because of a power failure. > > My question is this: How does Linux RAID5/RAID6 avoid reading an > entire stripe every time the kernel flushes a single page? Does it > have a (volatile?) cache? Or does it rely on the kernel flushing lots > of contiguous data in a single request? Or something else? This one I know... :-) There is a cache (volatile, since it is in system RAM), which can be tuned via sysfs. I've an i7 xeon with 12GiB RAM, 4 HDDs RAID-5 and I set the cache to 6GiB. This is dynamically allocated, so it uses RAM only when needed. Some benchmarks show that you can achieve the full 3 HDDs speed in small data writes and sustained write. I must say I was really impressed by the difference in writing performances after increasing the cache, not only in the benchmark world, but also with some I/O intensive applications. It made me rethink about the "quality" of the benchmarks you can find around: it seems nobody understood this capability of md. Of course, in case of power failure, without UPS, you risk a lot. Nevertheless, it depends on what are the overall requirements, I guess. > Does Linux RAID keep track of which disk blocks have already been > written at least once, so that there is a difference between writing a > block for the first time and updating it later? (But I guess that > would not make sense, since eventually all writes become updates as > files are created and deleted.) This one I do not know... :-) bye, -- piergiorgio -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
