Messages in this thread | | | Date | Mon, 22 Jun 2015 09:54:51 -0700 | Subject | Re: [PATCH 14/15] libnvdimm: support read-only btt backing devices | From | Dan Williams <> |
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On Mon, Jun 22, 2015 at 9:45 AM, Christoph Hellwig <hch@lst.de> 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-kernel" in Please read the FAQ at http://www.tux.org/lkml/
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