On 10/26/2014 06:43 PM, Dave Chinner wrote: > On Sat, Oct 25, 2014 at 12:35:17PM -0500, Stan Hoeppner wrote: >> If the same interface is used for Linux logical block devices (md, dm, >> lvm, etc) and hardware RAID, I have a hunch it may be better to >> determine that, if possible, before doing anything with these values. >> As you said previously, and I agree 100%, a lot of RAID vendors don't >> export meaningful information here. In this specific case, I think the >> RAID engineers are exporting a value, 1 MB, that works best for their >> cache management, or some other path in their firmware. They're >> concerned with host interface xfer into the controller, not the IOs on >> the back end to the disks. They don't see this as an end-to-end deal. >> In fact, I'd guess most of these folks see their device as performing >> magic, and it doesn't matter what comes in or goes out either end. >> "We'll take care of it." > > Deja vu. This is an isochronous RAID array you are having trouble > with, isn't it? I don't believe so. I'm pretty sure the parity rotates; i.e. standard RAID5/6. > FWIW, do your problems go away when you make you hardware LUN width > a multiple of the cache segment size? Hadn't tried it. And I don't have the opportunity now as my contract has ended. However the problems we were having weren't related to controller issues but excessive seeking. I mentioned this in that (rather lengthy) previous reply. >> optimal_io_size. I'm guessing this has different meaning for different >> folks. You say optimal_io_size is the same as RAID width. Apply that >> to this case: >> >> hardware RAID 60 LUN, 4 arrays >> 16+2 RAID6, 256 KB stripe unit, 4096 KB stripe width >> 16 MB LUN stripe width >> optimal_io_size = 16 MB >> >> Is that an appropriate value for optimal_io_size even if this is the >> RAID width? I'm not saying it isn't. I don't know. I don't know what >> other layers of the Linux and RAID firmware stacks are affected by this, >> nor how they're affected. > > yup, i'd expect minimum = 4MB (i.e stripe unit 4MB so we align to > the underlying RAID6 luns) and optimal = 16MB for the stripe width > (and so with swalloc we align to the first lun in the RAID0). At minimum 4MB how does that affect journal writes which will be much smaller, especially with a large file streaming workload, for which this setup is appropriate? Isn't the minimum a hard setting? I.e. we can never do an IO less than 4MB? Do other layers of the stack use this variable? Are they expecting values this large? > This should be passed up unchanged through the stack if none of the > software layers are doing other geometry modifications (e.g. more > raid, thinp, etc). I agree, if RAID vendors all did the right thing... Stan _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
