On Thu, May 30, 2019 at 06:08:53PM +0000, Andy Smith wrote: > Hi keld, > > Thanks for the reply. > > On Thu, May 30, 2019 at 12:04:20PM +0200, keld@xxxxxxxxxx wrote: > > you need to clarify which layout you use with md raid10. > > I did not bother as I included the commands for the array setup > which should indicate that default layout was used. yes it did. but it was hidden way down in the extended article. > > the layouts are near, far and offset, with very different performance characteristics. > > I did not think these would be of any interest on SSD/NVMe which is > my main concern and is the area where RAID-1 outperforms RAID-1 by a > factor of 3 for 100% 4KiB random reads. i think the latter raid-1 should read "md raid10,near". yes that is indeed strange, and probably due to the code being written with HDs in mind. > > > far and offset are designed to be faster than near, which I understand that you use. > > So why are you using the slowest md raid10 layout, and not mentioning this fact? > > Because I did not see the point of a non-default layout for fast > flash devices. i can understand your pow, but due to differences in the drivers it may actually matter. and maybe we can optimize the code a little for ssds. I have in mind some patches for the far layout, where the higher blocks are actually faster than the lower blocks. is this also true for ssds? > > maybe you could run your tests for all 3 layouts? > > Yes I will be happy to do this and see what happens but I'm not > optimistic that it will change matters so that RAID-10 is able to > direct most reads to the fastest half. which is the fastest half? does that apply to all ssds/nvme? keld
