On Wed, Nov 14, 2018 at 05:28:45PM +0100, Emmanuel Florac wrote: > Not a problem here: these are high end SN200 NVMe drives, with > capacitor-protected cache, caching a BBU-equipped RAID controller. Belt > and suspenders at the same time :) That is a safe setup indeed. > > Without a battery-backed RAM using a software RAID (md), Linux simply > > resorts to only sending one barrier operation at a time to the > > underlying drives plus additional housekeeping like a write intent > > bitmap. > > > > This fixes the consistency problem, however comes at a significant > > performance cost. Again, something you don't want with a cache device. > > > > So you're basically telling me that using md RAID 1 could possibly be > much slower than a Broadcom 94xx RAID controller for NVMe drives? > Interesting. It really depends on where the bottleneck is. But yes, it will be slower, because 'md' has no battery backed memory and thus has to limit the parallelism of the drives to achieve the same consistency guarantees. > > So, make your own decision based on your usecase. :) > > Use case is : as fast as possible, but doesn't endanger the 600 TB of > data, because duh, 600 TB is quite a lot :) As much as I try to eschew hardware RAID, and push md for the sake of openness, in this case I believe it's the right solution for protecting against the NVMe SSD failure. -- Vojtech Pavlik Director SUSE Labs
