Supporting human parsable journal_dev specifiers

Supporting human parsable journal_dev specifiers

[Theodore Ts'o]

I have a user request that mount be able to interpret the device
specifier for the journal_dev mount option, so that instead of having
to type something like

   mount -o journal_dev=0x0803 ....

they could unstead use: "journal_dev=/dev/sda3" or even
"journal_dev=LABEL=ext_journal".  I assume the right way to do this is
a mount helper?  Or is this something that you would be willing to
accept as a patch into util-linux?

If it is a mount helper, what's the best way to do things?  Simply
parse through the mount options until finding the journal_dev one, and
then calling blkid or stat as necessary to interpret the argument, and
then re-exec mount with the modified mount option?  Or is there some
gotchas involved with doing this?

Thanks,

						- Ted

Re: Supporting human parsable journal_dev specifiers

[Christoph Hellwig]

On Thu, Jul 10, 2014 at 10:28:24AM -0400, Theodore Ts'o wrote:
> I have a user request that mount be able to interpret the device
> specifier for the journal_dev mount option, so that instead of having
> to type something like
> 
>    mount -o journal_dev=0x0803 ....
> 
> they could unstead use: "journal_dev=/dev/sda3" or even
> "journal_dev=LABEL=ext_journal".  I assume the right way to do this is
> a mount helper?  Or is this something that you would be willing to
> accept as a patch into util-linux?

At least for the device name the best is to implement it in the kernel,
take a look at what XFS is doing for the log device.  The open by dev_t
functionality currently used by extN isn't something we like to see used
anyway.

Re: Supporting human parsable journal_dev specifiers

[Karel Zak]

On Thu, Jul 10, 2014 at 10:28:24AM -0400, Theodore Ts'o wrote:
> I have a user request that mount be able to interpret the device
> specifier for the journal_dev mount option, so that instead of having
> to type something like
> 
>    mount -o journal_dev=0x0803 ....
> 
> they could unstead use: "journal_dev=/dev/sda3" or even
> "journal_dev=LABEL=ext_journal".  I assume the right way to do this is
> a mount helper?  Or is this something that you would be willing to
> accept as a patch into util-linux?

 I talked about it with Eric Sandeen year ago, and the decision was to
 implement some generic FS independent solution based on variables in
 mount options, something like

     mount -o journal_dev=$(LABEL:<ext_journal_label>)

 (or $LOGUUID for XFS log, etc).

 It's not implemented yet, but I can try it if it's still necessary.
 
> If it is a mount helper, what's the best way to do things?  Simply

 Please, do not use mount helpers (/sbin/mount.<type>), it sucks in
 many ways... it will be better to have a generic solution within
 mount(8).

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com