On Thursday October 5, madduck@xxxxxxxxxxx wrote: > I am trying to compare the three RADI10 layouts with each other. > Assuming a simple 4 drive setup with 2 copies of each block, > I understand that a "near" layout makes RAID10 resemble RAID1+0 > (although it's not 1+0). > > I also understand that the "far" layout trades some read performance > for some write performance, so it's best for read-intensive > operations, like read-only file servers. > > I don't really understand the "offset" layout. Am I right in > asserting that like "near" it keeps stripes together and thus > requires less seeking, but stores the blocks at different offsets > wrt the disks? > > If A,B,C are data blocks, a,b their parts, and 1,2 denote their > copies, the following would be a classic RAID1+0 where 1,2 and 3,4 > are RAID0 pairs combined into a RAID1: > > hdd1 Aa1 Ba1 Ca1 > hdd2 Ab1 Bb1 Cb1 > hdd3 Aa2 Ba2 Ca2 > hdd4 Ab2 Bb2 Cb2 > > How would this look with the three different layouts? I think "near" > is pretty much the same as above, but I can't figure out "far" and > "offset" from the md(4) manpage. near=2 would be hdd1 Aa1 Ba1 Ca1 hdd2 Aa2 Ba2 Ca2 hdd3 Ab1 Bb1 Cb1 hdd4 Ab2 Bb2 Cb2 offset=2 would be hdd1 Aa1 Bb2 Ca1 Db2 hdd2 Ab1 Aa2 Cb1 Ca2 hdd3 Ba1 Ab2 Da1 Cb2 hdd4 Bb1 Ba2 Db1 Da2 far=2 would be hdd1 Aa1 Ca1 .... Bb2 Db2 hdd2 Ab1 Cb1 .... Aa2 Ca2 hdd3 Ba1 Da1 .... Ab2 Cb2 hdd4 Bb1 Db1 .... Ba2 Da2 Where the second set start half-way through the drives. The advantage of far= is that you can easily spread a long sequential read across the drives. The cost is more seeking for writes. offset= can possibly get similar benefits with large enough chunk size, though I haven't tried to understand all the implications of that layout. I added it simply because it is a supported layout in DDF and I am working towards DDF support. NeilBrown - 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
