On Wed, 30 May 2007, Jon Nelson wrote: > On Thu, 31 May 2007, Richard Scobie wrote: > > > Jon Nelson wrote: > > > > > I am getting 70-80MB/s read rates as reported via dstat, and 60-80MB/s as > > > reported by dd. What I don't understand is why just one disk is being used > > > here, instead of two or more. I tried different versions of metadata, and > > > using a bitmap makes no difference. I created the array with (allowing for > > > variations of bitmap and metadata version): > > > > This is normal for md RAID1. What you should find is that for > > concurrent reads, each read will be serviced by a different disk, > > until no. of reads = no. of drives. > > Alright. To clarify, let's assume some process (like a single-threaded > webserver) using a raid1 to store content (who knows why, let's just say > it is), and also assume that the I/O load is 100% reads. Given that the > server does not fork (or create a thread) for each request, does that > mean that every single web request is essentially serviced from one > disk, always? What mechanism determines which disk actually services the > request? It's probably bad form to reply to one's own posts, but I just found static int read_balance(conf_t *conf, r1bio_t *r1_bio) in raid1.c which, if I'm reading the rest of the source correctly, basically says "pick the disk whose current head position is closest". This *could* explain the behavior I was seeing. Is that not correct? -- Jon Nelson <jnelson-linux-raid@xxxxxxxxxxx> - 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
