Re: [PATCH] xfs: fail dax mount if reflink is enabled on a partition

Yang, Xiao/杨 晓
2022-09-14T06:44:24Z

On 2022/9/9 21:01, Brian Foster wrote:
> Yes.. I don't recall all the internals of the tools and test, but IIRC
> it relied on discard to perform zeroing between checkpoints or some such
> and avoid spurious failures. The purpose of running on dm-thin was
> merely to provide reliable discard zeroing behavior on the target device
> and thus to allow the test to run reliably.

Hi Brian,

As far as I know, generic/470 was original designed to verify
mmap(MAP_SYNC) on the dm-log-writes device enabling DAX. Due to the
reason, we need to ensure that all underlying devices under
dm-log-writes device support DAX. However dm-thin device never supports
DAX so
running generic/470 with dm-thin device always returns "not run".

Please see the difference between old and new logic:

old logic new logic
---------------------------------------------------------------
log-writes device(DAX) log-writes device(DAX)
| |
PMEM0(DAX) + PMEM1(DAX) Thin device(non-DAX) + PMEM1(DAX)
|
PMEM0(DAX)
---------------------------------------------------------------

We think dm-thin device is not a good solution for generic/470, is there
any other solution to support both discard zero and DAX?

BTW, only log-writes, stripe and linear support DAX for now.

Best Regards,
Xiao Yang