Allow to specify a fs type for a vnode-backed memory disk

[ak]

Summary

Allow to specify a file system type for a vnode-backed memory disk.

This will simplify mounting images from command line:

#mdmfs -T cd9660 -P -F foo.iso md /mnt/cdrom
instead of
#mdconfig -a -t vnode -f foo.iso -u 10000
#mount_cd9660 /dev/md10000 /mnt/cdrom

And allows to mount images from /etc/fstab:
echo 'md /media mfs rw,-Tmsdosfs,-P,-F/path/image.img 0 0' >> /etc/fstab

Test Plan

Apply the patch, rebuild mdmfs and mount/unmount some images.

[wblock]

sbin/mdmfs/mdmfs.8

234

Is there a line missing here? (I know, it's not part of your patch.)

243

Why not UFS?

244

.Fl T
only applies when
.Fl F
and
.Fl P
are used.

[ak]

sbin/mdmfs/mdmfs.8

234

Actually, I'd implemented that option too. I think it should had been "on created filesystem which allows return of". This sentence is a little awkward, so I will be grateful for any suggestions.

243

Sure, you can specify -T ufs, but ufs is default, you don't need -T to select it.

244

Noted.

[wblock]

sbin/mdmfs/mdmfs.8

234

Line 231 has a "The", which I think can just be deleted. Then for the rest:

.Xr md 4
supports the BIO_DELETE command, also known as TRIM.
When used with a filesystem that supports TRIM,
.Xr md 4
returns freed blocks to the system memory pool.

[ak]

sbin/mdmfs/mdmfs.8

234

This is slightly technically incorrect: BIO_DELETE is a geom command, which indicates that a certain range of data is no longer used and can be erased, trim is an implementation specific for ata, for scsi, for example, it is called unmap, so they are not equivalent in general. Also trim on fs needs to be explicitly enabled, while md(4) supports it passively, so IMHO previous sentence was a little more clear.

[imp]

sbin/mdmfs/mdmfs.8

234

I'd remove the 'aka TRIM bit, and then say

When used with a filesystem that issue BIO_DELETE bio requests,
.Xr md 4
returns deleted blocks to the system memory pool.

or

.Xr md 4 returns blocks to the system memory pool in response
to BIO_DELETE commands.

[ak]

• Bump manpage's date
• Rephrase -t option description
• Clarify -T option usage

[wblock]

One note, but the man page looks good.

head/sbin/mdmfs/mdmfs.8

311 ↗

(On Diff #11186)

Please just split the sentence here:

with the same letter.
The
.Fl T

[ak]

• Split long sentences.

[mav]

I generally have no objections against adding another flag. It looks useful. But I have couple related comments.

head/sbin/mdmfs/mdmfs.c

514 ↗

(On Diff #11507)

Why only those file systems, and why this tool should at all bother what file system types are supported by OS/mount tool?

sbin/mdmfs/mdmfs.8

243

Default may change at some point, who knows, and it is good to be able to explicitly specify things even if they are default. Inability to do it create more problems for other code that has also know about this UFS default, and that this options must not be specified in case of UFS.

[ak]

• Don't check for valid fstype, rely on mount(8) for this
• Update -T option description

[mav]

Looks good to me.