Chan Chung Hang Christopher schrieb: >> "md1 will read from both disk" is not true in general. >> RAID1 md reads from one disk only; it uses the other one in case the >> first one fails. No performance gain from multiple copies. >> > I beg to differ. I have disks in a raid1 md array and iostat -x 1 will > show reads coming off both disks. Unless you do not have the multipath look more carefully - with the current 2.6.18-9.1.22 kernel the bulk of the data are read from one of the disks > module loaded, md will read off both disks. Now whether md will read > equally off both disks, that certainly will not be true in general. >> You can easily see this for yourself by setting up a RAID1 from e.g. >> sda1 and sdb1 - /proc/mdstat is: >> >> Personalities : [raid1] >> md1 : active raid1 sdb1[1] sda1[0] >> 104320 blocks [2/2] [UU] >> >> and then comparing the output of hdparm -tT : >> > ROTFL. > > How about using the proper tool (iostat) and generating some disk load > instead? hdparm -tT tests one type of disk access, other tools test other aspects. I gave the hdparm numbers because everyone can reproduce them. For RAID0 with two disks you do see - using e.g. hdparm - the doubling of performance from two disks. If you take the time to read (or do) RAID benchmarks you'll discover that Linux software RAID1 is about as fast as a single disk (and RAID0 with two disks is about twice the speed). It's as simple as that. > >> To get performance gain in RAID1 mode you need hardware RAID1. >> > > Bollocks. The only area in which hardware raid has a significant > performance advantage over software raid is raid5/6 given sufficient > cache memory and processing power. We were talking about RAID1; RAID5/6 is a different area. Linux software RAID1 is a safeguard against disk failure; it's not designed for speed increase. There is a number of things that could be improved in Linux software RAID; read performance of RAID1 is one of them - this _is_ why some hardware RAID1 adapters indeed are faster than software. Read http://kernelnewbies.org/KernelProjects/Raid1ReadBalancing - since the 2.6.25 kernel a simple alternating read is implemented, but that does not take the access pattern into account. So Linux software RAID1 is just mirroring - and it's good at that. HTH, Kay