[NFS] Reflect new Sync default in export options

[FroggyFlox]

The manpage for exports reads:

In releases of nfs-utils up to and including 1.0.0, the async option was the default. In all releases after 1.0.0, sync is the default, and async must be explicitly requested if needed. To help make system administrators aware of this change, exportfs will issue a warning if neither sync nor async is specified.

https://linux.die.net/man/5/exports

Leap 15.3 currently has nfs-utils v2.1.1:

# zypper info nfs-utils
Loading repository data...
Reading installed packages...


package 'nfs-utils' not found.

Information for srcpackage nfs-utils:
-------------------------------------
Repository            : Update repository with updates from SUSE Linux Enterprise 15
Name                  : nfs-utils
Version               : 2.1.1-150100.10.24.1
Arch                  : noarch
Vendor                : SUSE LLC <https://www.suse.com/>
Summary               : Support Utilities for Kernel nfsd
Description           : 
    This package contains the NFS utilities. You can tune the number of
    server threads via the sysconfig variable USE_KERNEL_NFSD_NUMBER. For
    quota over NFS support, install the quota package.
Builds binary package : 
    S | Name              | Version
    --+-------------------+---------------------
    i | nfs-client        | 2.1.1-150100.10.24.1
      | nfs-doc           | 2.1.1-150100.10.24.1
    i | nfs-kernel-server | 2.1.1-150100.10.24.1

Currently. our docs incorrectly list the "sync" option as default (will be corrected in the fix for rockstor/rockstor-doc#345), and the webUI reflects the same, with the async choice being the default selected option.

We should follow the nfs defaults and make the sync choice as selected by default.

[FroggyFlox]

We might want to take advantage of this issue to also provide links to our docs related to NFS; these would be the various sections of https://rockstor.com/docs/interface/storage/file_sharing/nfs_ops.html.

[FroggyFlox]

Note that this might also require a small update of our NFSExport model:

rockstor-core/src/rockstor/storageadmin/models/nfs_export_group.py

Lines 49 to 53 in 578c53e

	"""mount async by default"""
	SYNC_CHOICES = (
	(ASYNC, "async"),
	(SYNC, "sync"),
	)

[phillxnet]

Linking to recent attention the NFS side of things has received:

• NFS tests in need of maintenance NFS tests in need of maintenance #2923 NFS tests in need of maintenance #2923 #2925
• Inactive NFS export validation Inactive NFS export validation #2924 Inactive NFS export validation #2924 #2926
• NFS exports restored as read-only NFS exports restored as read-only #2912 NFS exports restored as read-only #2912 #2927

As of now Leap 15.6 has the following info shown for the source package nfs-utils:

rleap15-6:~ # zypper info nfs-utils
Loading repository data...
Reading installed packages...


package 'nfs-utils' not found.

Information for srcpackage nfs-utils:
-------------------------------------
Repository            : Update repository with updates from SUSE Linux Enterprise 15
Name                  : nfs-utils
Version               : 2.6.4-150600.28.3.2
Arch                  : noarch
Vendor                : SUSE LLC <https://www.suse.com/>
Summary               : Support Utilities for Kernel nfsd
Description           : 
    This package contains the NFS utilities. You can tune the number of
    server threads via the sysconfig variable USE_KERNEL_NFSD_NUMBER. For
    quota over NFS support, install the quota package.
Builds binary package : 
    S | Name              | Version
    --+-------------------+--------------------
      | libnfsidmap0      | 0.26-150600.28.3.2
    i | libnfsidmap1      | 1.0-150600.28.3.2
    i | nfs-client        | 2.6.4-150600.28.3.2
      | nfs-doc           | 2.6.4-150600.28.3.2
    i | nfs-kernel-server | 2.6.4-150600.28.3.2
      | nfsidmap-devel    | 1.0-150600.28.3.2
      | nfsidmap0-devel   | 0.26-150600.28.3.2

I'm a little reluctant to approach this change in our now late testing phase: given the model change requirement. But at least our tests have now seen some recent maintenance.

I've added the new UI-enhancement label to this issue so we can more easily pick it out for consideration during the next testing phase; given we are to focus more on the front-end, albeit the model is a back-end change for this one. But we will also have some front end changes here to modify this default. However we also need to reflect this change in our API interface defaults: assuming this default change in sync/async is still the prevalent thought. I agree however that we should reflect the safer approach taken by the upstream here. But as recent forum posts have indicated, sync can have sever performance drawbacks. We could perhaps indicate this in our Web-UI changes while we are there changing this default.

[phillxnet]

Copying in a more modern reference to this same nfs export group model:

rockstor-core/src/rockstor/storageadmin/models/nfs_export_group.py

Lines 45 to 55 in 5d0525e

	"""mount async by default"""
	SYNC_CHOICES = (
	(ASYNC, "async"),
	(SYNC, "sync"),
	)
	syncable = models.CharField(
	max_length=5,
	choices=SYNC_CHOICES,
	default=ASYNC,
	validators=[validators.validate_nfs_sync_choice],
	)

Essentially unchanged, but we see the default setting further down in the above.