Mounting a filesystem on a DCSS with DAX support

7.1 z/VM guest

Linux Direct Access (DAX) enables filesystems on directly accessible, memory‑like block devices to bypass the Linux page cache.

Before you begin

DAX support is only available for DCSS devices with 2 MB aligned start address and size, and only for ext2, ext4 and xfs filesystems.

About this task

This functionality is equivalent to the former XIP (execute‑in‑place) support for DCSS, which was removed from the kernel. Similar to the XIP support for DCSS, this could be useful to reduce Linux z/VM guest memory footprint, when using large binaries on possibly shared DCSS devices. Mounting a filesystem on a DCSS device with the -o dax option now provides the same capabilities as the earlier -o xip option.

In addition, DAX supports ext2, ext4, and XFS filesystems in both read‑only and read‑write modes, whereas XIP was limited to read‑only ext2.

Procedure

To enable DAX support, specify the "dax" mount option:

# mount -o dax /dev/dcssblk0 /mnt

For more details, see the documentation file Documentation/filesystems/dax.rst in the kernel source.