The future of DAX

Posted Mar 29, 2017 2:44 UTC (Wed) by dgc (subscriber, #6611)
Parent article: The future of DAX

> Another wishlist feature is support for 1GB extra-huge pages [....] would require filesystems to lay out
> files in 1GB-aligned extents, which none do now.

Correction: XFS has supported 1GB aligned extents for about 15 years now. It uses the same mechanism - extent size hints - that is being used to guarantee 2MB aligned extents for supporting huge pages with DAX on XFS. I've mentioned this during discussions several times over the past couple of years....

The future of DAX

Posted Mar 29, 2017 3:56 UTC (Wed) by rzwisler (subscriber, #90544) [Link] (2 responses)

Interesting...when Matthew last tried to get it working (over a year ago), I believe he couldn't get a 1GiB sized and aligned extent in his setup. This made us look at realtime devices, which also didn't work with DAX. If XFS already supports this and we just needed a different configuration, this makes the lift for 1 GiB support in FS DAX much lighter.

The future of DAX

Posted Mar 29, 2017 10:59 UTC (Wed) by dgc (subscriber, #6611) [Link]

I think realtime devices should work now that the iomap infrastructure feeds dax the block device it needs to map. I haven't tested it, but a quick glance at the code says it should work.

FWIW, files in the realtime device are aligned just like normal files except they have a fixed minimum extent size hint that is set on the rt device at mkfs time (see xfs_get_extsz_hint()). IOWs, it uses the same mechanism for aligned allocations that we're supposedly already using for huge pages with DAX..... :P

The future of DAX

Posted Mar 30, 2017 19:55 UTC (Thu) by willy (subscriber, #9762) [Link]

I wasn't trying with XFS but with ext4.