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? 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.) Thanks. - Pat -- 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
