On 5 June 2012 07:14, Ole Tange <ole@xxxxxxxx> wrote: > On my new 24 disk array I get 900 MB/s of raw read or write using `dd` > to all the disks. — Array of layout what? > When I set the disks up as a 24 disk software RAID6 I get 400 MB/s > write and 600 MB/s read. It seems to be due to checksuming, as I have > a single process (md0_raid6) taking up 100% of one CPU. […] > I tested this by creating 24 devices in RAM, used different chunk > sizes, and then copied the linux kernel source. Test script can be > found on http://oletange.blogspot.dk/2012/05/software-raid-performance-on-24-disks.html What a wild train of thoughts… Are those 24 disks HDDs or they're "in RAM"? > By doing it in RAM the results are not affected by physical disks or > disk controller. So the only change is the speed of computing > checksums. This can also be seen as the time the process md0_raid0 is > running. > > The results were: > > Chunk size Time to copy 10 linux kernel sources as files Time to copy > 10 linux kernel sources as a single tar file > 16 32s 13s […] > 4096 1m38s 16s You were talking bout MB/secs and now you're not. It doesn't help understanding you either. > It makes sense that it is faster to copy 10 big files than 10 times […] > > But I cannot explain why even the best performance (4600 MB/11s = 420 > MB/s) is not even close to the checksum performance reported by the > kernel at boot (6196 MB/s): > > Mar 13 16:02:42 server kernel: [ 35.120035] raid6: using > algorithm sse2x4 (6196 MB/s) > > Can you explain why I only get 420 MB/s of real world checksumming > instead of 6196 MB/s? Again — 420 MB/sec on HDD-based RAID or in-RAM one? What do you think LSR subscribers are — mediums? -- -- 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
