On Mon, Oct 27, 2014 at 06:04:05PM -0500, Stan Hoeppner wrote: > 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. The location of parity doesn't dtermine that it is isochronous in behaviour or not. Often RAID5/6 is marketing speak for "single/dual parity", not the type of redundancy that is implemented in the hardware ;) > > 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. Right, but if you had a 768k stripe width and a 1MB cache segment size, a cache segment operation would require two stripe widths to be operated on, and only one would be a whole stripe width. hence the possibility of doing more IOs than are necessary to populate or write back cache segments. i.e. it's a potential reason for why the back end disks didn't have anywhere near the expected seek capability they were supposed to have.... > >> 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? No, "minimum_io_size" is for "minimum *efficient* IO size" not the smallest supported IO size. The smallest supported IO sizes and atomic IO sizes are defined by hw_sector_size, physical_block_size and logical_block_size. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
