need auto-detection of directio alignment requirements

[carns]

In GitLab by @carns on Apr 7, 2020, 13:40

Some systems need 512 bytes, others need 4096. It's not obvious how to tell ahead of time which is required. In the worst case we could loop over a few options until a write succeeds when creating the volume and loop over read on adding a target.

[carns]

In GitLab by @carns on Apr 7, 2020, 13:55

Pointed out by @roblatham00 (this is from man pages):

       ical block size of the underlying storage (typically 512 bytes) suffices.  The logical block size can be determined using the ioctl(2) BLKSSZGET operation or from the shell using the command:
           blockdev --getss```

[carns]

In GitLab by @carns on Apr 7, 2020, 14:23

The above requires permission to open the block device, which isn't commonly available with normal user privs. Will likely need to just probe read/write sizes in practice.