On Mon, Jun 22, 2015 at 9:45 AM, Christoph Hellwig <hch@xxxxxx> wrote: > On Mon, Jun 22, 2015 at 09:36:50AM -0700, Dan Williams wrote: >> In that case "don't stack" is too coarse of a hammer. I see this as a >> request to hide the subordinate ULD which is a new capability that DM >> and MD might benefit from as well. We already have the case in MD >> where it internally holds a reference to bdev that has been hot >> removed, it seems not much of a stretch to have stacking drivers be >> able to hide device nodes for bdevs that they are holding. > > I don't see why you're comparing with MD and DM here. MD and DM > sit cleanly ontop of any block device. If btt was independent of > libnvdimm and just used ->rw_bytes we could see it as this. > > But it's all a giant entangled mess, where btt for example is probed > by libnvdimm. At the same time pmem.c isn't really a true block > driver, it's really just a trivial shim between the block API > and pmem-style memcpy. Especially with the proper pmem API btt > would become cleaner just calling that directly. The pmem api does nothing to fix torn sectors, there's no extra atomicity guarantees that come from those instructions. >> Yes, if they want to use DAX they should do it consciously and audit >> their application to be sure it is safe to abandon atomic sector >> guarantees. With the current flexibility to do BTT on a partition >> they can do this conversion piecemeal and, for example, keep metadata >> on BTT and data on DAX. > > By that logic you'd want to attach BTT by default and allow opt-out > at some level. This could be a libnvmdimm-level partitioning scheme, > which would also allow storing the bit if BTT is used or not persistently. > Or it could be on fine grained boundaries which might be more useful. Well, let's start with per-disk btt and see where that gets us, we can always ramp up complexity later. I'd just as soon make the default opt-in/out a Kconfig toggle with a sysfs override. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
