On Thu, 2005-07-14 at 13:54 -0400, Dan Christensen wrote: > Ming Zhang <mingz@xxxxxxxxxxx> writes: > > > On Wed, 2005-07-13 at 23:58 -0400, Dan Christensen wrote: > > > >> But I still don't understand why the md layer can't do better. If I > >> turn off readahead of the raw devices, and keep it for the raid > >> device, then parity blocks should never be requested, so they > >> shouldn't use any bus/controller bandwidth. And even if each drive is > >> only acting at 75% efficiency, the four drives should still be able to > >> saturate the bus/controller. So I can't figure out what's going on > >> here. > > > > when read, i do not think MD will read parity at all. but since parity > > is on all disk, there might be a seek here. > > Yes, there will be a seek, or internal drive readahead, so each drive > will operate at around 75% efficiency. But since that shouldn't > affect bus/controller traffic, I still would expect to get over > 100MB/s with my hardware. agree. but what if your controller is a bottleneck? u need to have another card to find out. > > >> Also, is there a way to disable caching of reads? > > > > after you run your code, check the meminfo, the cached value might be > > much lower than u expected. my feeling is that linux page cache will > > discard all cache if last file handle closed. > > Ming Zhang <mingz@xxxxxxxxxxx> writes: > > > my problem here. this only apply to sdX not mdX. pls ignore this. > > I'm not sure what you mean. For reads from sdX, mdX, files on sdX > or files on mdX, the cache is retained. So it's necessary to clear > this cache to get valid timing results. yes, i was insane at that time, pls ignore these blah blah. > > Dan > > - > 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 - 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
