On Mon, Aug 26, 2024 at 12:14:04PM -0700, Darrick J. Wong wrote: > They're not sparse like fsbnos on the data device, they're laid end to > end. IOWs, it's a straight linear translation. If you have an rtgroup > that is 50 blocks long, then rtgroup 1 starts at (50 * blocksize). Except with the zone capacity features on ZNS devices, where they already are sparse. But that's like 200 patches away from the state here.. > group 0 on a !rtg filesystem can be 64-bits in block/rt count. This is > a /very/ annoying pain point -- if you actually created such a > filesystem it actually would never work because the rtsummary file would > be created undersized due to an integer overflow, but the verifiers > never checked any of that, and due to the same underflow the rtallocator > would search the wrong places and (eventually) fall back to a dumb > linear scan. > > Soooooo this is an obnoxious usecase (broken large !rtg filesystems) > that we can't just drop, though I'm pretty sure there aren't any systems > in the wild. So, do we really need to support that? I think we've always supported a 64-bit block count, so we'll have to support that, but if a > 32bit extent count was always broken maybe we should simply stop to pretend to support it? > > What's the maximum valid rtg number? We're not ever going to be > > supporting 2^32 - 2 rtgs, so what is a realistic maximum we can cap > > this at and validate it at? > > /me shrugs -- the smallest AG size on the data device is 16M, which > technically speaking means that one /could/ format 2^(63-24) groups, > or order 39. > > Realistically with the maximum rtgroup size of 2^31 blocks, we probably > only need 2^(63 - (31 + 10)) = 2^22 rtgroups max on a 1k fsblock fs. Note that with zoned file system later on we are bound by hardware size. SMR HDDs by convention some with 256MB zones. This is a bit on the small side, but grouping multiple of those into a RT group would be a major pain. I hope the hardware size will eventually increase, maybe when they move to 3-digit TB capcity points.
