[stgt] Late opening of the backing store

[Môshe van der Sterre]

(Oops, I clicked the wrong reply button)

Hi Fujita,


On Thu, 19 Jan 2012 06:31:19 +0900, FUJITA Tomonori
<fujita.tomonori at lab.ntt.co.jp> wrote:
> **UNKNOWN CHARSET** <me at moshe.nl> wrote:
> 
>> When a LUN is created with tgtd, tgtd immediately open()s the backing
>> store, however I don't think data is actually read before a connection
>> from an initiator is made.
>>
>> My question is, is there an option to make tgtd open the file the
>> moment a connection is made (and not before)?
>> And if this is not currently possible, what would it take to implement
>> this?
>> As a related feature, the backing store could be close()d after the
>> (last) connection has ended.
> 
> tgt enables you to add/remove logical units any time. Why you can't
> add a device such as lvm volumes when it's ready?

Adding works fine. The problem is, the moment it is added, the
file/volume/disk is 'in use' (by stgt), this prevents (for example) the
lvm tools from changing or removing the device.

For example, I want the ability to export all volumes in a LVM
volumegroup automatically, this can easily be done with a udev script
to
add the LUN. The same could be done for removing the volume, only stgt
prevents this by making LVM mark the device 'in use'. I would like a
option so stgt only 'uses' the device, if there is a connection that
_really_ uses the device.

Is short this means stgt should defer the open() call on the backing
store until a connection is requested. Likewise, it should close() the
backing store when the last connection closes.
I am guessing such a feature does not yet exist (I couldn't find it),
and am interested to know your guess on how hard this would be to
implement.

If there are any technical arguments against such a feature, or it
requires a lot of changes to stgt, I should investigate alternative
ways
to manage the volumes (by using administration tools that allow hooks
to
remove the volume from stgt before trying to change it).

Greetings,
Môshe van der Sterre

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