mount on the same mount point

climb65 wrote:
> Hello,
>
> I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
> mount point. I have just read that it is possible.
>
> Note : MD devices are JFS.
>
> But, altough I get no error, I do not get what I expected : the second
> mount command makes the first mount invisible.
>
> So, my 2 questions are :
>
> 1) is it possible to mount 2 MD devices on the same mount point?

yes, but the results are as above.

> 2) If yes, how?

As you did it.

If what you want is for two disks to apparently 'share' the same flat
directory space, then symlinks are possibly your friend.


>
> Thanks

climb65 <climb65.DeleteThis@laposte.net> wrote:
> I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
> mount point. I have just read that it is possible.

Yes it's possible.


> But, altough I get no error, I do not get what I expected : the second
> mount command makes the first mount invisible.

That's exactly what should happen.


> 1) is it possible to mount 2 MD devices on the same mount point?
> 2) If yes, how?

Yes it is, by doing what you're doing. But it sounds like your expectation
differs from reality, and it would help somewhat if you were to explain
what you were actually trying to achieve.

Chris

On Fri, 09 Oct 2009 10:49:47 +0000, climb65 wrote:

> Hello,
>
> I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
> mount point. I have just read that it is possible.
>
> Note : MD devices are JFS.
>
> But, altough I get no error, I do not get what I expected : the second
> mount command makes the first mount invisible.
>
> So, my 2 questions are :
>
> 1) is it possible to mount 2 MD devices on the same mount point? 2) If
> yes, how?
>
> Thanks

Let's think about this for a second. If you did as you wanted and you
tried to access a particular file - suppose there was a file on each disk
with the same name - how would the system know which one you wanted? If
you created a file in the common directory - where would it be stored?

On Oct 9, 11:14 am, ray <r....DeleteThis@zianet.com> wrote:
> On Fri, 09 Oct 2009 10:49:47 +0000, climb65 wrote:
> > Hello,
>
> > I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
> > mount point. I have just read that it is possible.
>
> > Note : MD devices are JFS.
>
> > But, altough I get no error, I do not get what I expected : the second
> > mount command makes the first mount invisible.
>
> > So, my 2 questions are :
>
> > 1) is it possible to mount 2 MD devices on the same mount point? 2) If
> > yes, how?
>
> > Thanks
>
> Let's think about this for a second. If you did as you wanted and you
> tried to access a particular file - suppose there was a file on each disk
> with the same name - how would the system know which one you wanted? If
> you created a file in the common directory - where would it be stored?

What he wants is a "union mount". See my http://aplawrence.com/Words/2004_11_21.html

--
Tony Lawrence

On Oct 9, 11:14 am, ray <r....RemoveThis@zianet.com> wrote:

> Let's think about this for a second. If you did as you wanted and you
> tried to access a particular file - suppose there was a file on each disk
> with the same name - how would the system know which one you wanted? If
> you created a file in the common directory - where would it be stored?

The ramifications of union mounts can be confusing.

The system doesn't know which one you wanted - but it does know which
one you'll get

A more interesting question is what happens when a file in the upper
mount is deleted but a file with the name exists below. Should the
"under" file become visible? The answer depends upon whether or not
"whiteouts" are implemented.

See http://aplawrence.com/foo-mac/union-mounts.html for an example
(which happens to be on Mac OS X but the same concepts apply to Linux
union mounts).

On Fri, 9 Oct 2009, climb65 wrote:

> Hello,
>
> I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
> mount point. I have just read that it is possible.
>
What exactly are you trying to do?

The minute you mount something to a mount point, whatever is there
disappears until the mounted partition is unmounted (and then it
reappears unharmed). A mount point is merely a directory, if it's empty
it means nothing to mount something there, if it's got content the content
disappears until there is a umount.

One can mount on a subdirectory of a mount point.

I have /home in a separate partition. When I move to a new release of my
distribution, I put it in a new partition, and play with it there for a
while, in tandem with the previous release. When I'm ready to make the
changeover, I make /home in the new release's partition the mount point
for my separate /home partition.

But, I also have a directory in my /home directory titled "mnt", where I
can mount flash drives and such. So I have a mount point within a mounted
partition.

You can do that, mount one partition, create a directory where you can
mount the next, and once the first is mounted, you mount the second.
As someone pointed out, this also ensures that you can tell which
content is in which partition, and if there are duplicates there won't
be a clash.

Michael


> Note : MD devices are JFS.
>
> But, altough I get no error, I do not get what I expected : the second
> mount command makes the first mount invisible.
>
> So, my 2 questions are :
>
> 1) is it possible to mount 2 MD devices on the same mount point?
> 2) If yes, how?
>
> Thanks
>

On Oct 9, 1:04 pm, Michael Black <et... DeleteThis @ncf.ca> wrote:
> On Fri, 9 Oct 2009, climb65 wrote:
> > Hello,
>
> > I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
> > mount point. I have just read that it is possible.
>
> What exactly are you trying to do?

What he's trying to do is obviously a union mount.

Why does everybody keep insisting this can't be done?

http://www.filesystems.org/project-unionfs.html

On Fri, 09 Oct 2009 10:49:47 +0000, climb65 wrote:

> Hello,
>
> I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
> mount point. I have just read that it is possible.
>
> Note : MD devices are JFS.
>
> But, altough I get no error, I do not get what I expected : the second
> mount command makes the first mount invisible.

It worked correctly

>
> So, my 2 questions are :
>
> 1) is it possible to mount 2 MD devices on the same mount point? 2) If
> yes, how?
>

No

--
GNU/Linux runs on IBM mainframes and on the world's fastest supercomputers
Windows supercomputers on the other hand are called botnets. <grin>

On Fri, 09 Oct 2009 11:10:13 -0700, Tony Lawrence (pcunix) wrote:

> On Oct 9, 1:04 pm, Michael Black <et....TakeThisOut@ncf.ca> wrote:
>> On Fri, 9 Oct 2009, climb65 wrote:
>> > Hello,
>>
>> > I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
>> > mount point. I have just read that it is possible.
>>
>> What exactly are you trying to do?
>
> What he's trying to do is obviously a union mount.
>
> Why does everybody keep insisting this can't be done?
>
> http://www.filesystems.org/project-unionfs.html


What if unionfs is not available?

How would you make it work then?


--
GNU/Linux runs on IBM mainframes and on the world's fastest supercomputers
Windows supercomputers on the other hand are called botnets. <grin>

Baho Utot <baho-utot.RemoveThis@invalid.com> writes:

>On Fri, 09 Oct 2009 10:49:47 +0000, climb65 wrote:

>> Hello,
>>
>> I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
>> mount point. I have just read that it is possible.
>>
>> Note : MD devices are JFS.
>>
>> But, altough I get no error, I do not get what I expected : the second
>> mount command makes the first mount invisible.

>It worked correctly

On a regular mount, whatever is mounted on a mountpoint hides everything
that was previously on that mount point. The former stuff is
inaccessible.

As people have pointed out there is a set of routines, called Union File
System, by which you can mount inclusively-- ie all the material mounted
as a union file system is accessible.

Read http://en.wikipedia.org/wiki/UnionFS
Linux has both unionfs and aufs as implimentations.



>>
>> So, my 2 questions are :
>>
>> 1) is it possible to mount 2 MD devices on the same mount point? 2) If
>> yes, how?
>>

>No

Baho Utot <baho-utot.DeleteThis@invalid.com> writes:

>On Fri, 09 Oct 2009 11:10:13 -0700, Tony Lawrence (pcunix) wrote:

>> On Oct 9, 1:04 pm, Michael Black <et....DeleteThis@ncf.ca> wrote:
>>> On Fri, 9 Oct 2009, climb65 wrote:
>>> > Hello,
>>>
>>> > I'm trying to mount 2 md devices (/dev/md4 and /dev/md5) on the same
>>> > mount point. I have just read that it is possible.
>>>
>>> What exactly are you trying to do?
>>
>> What he's trying to do is obviously a union mount.
>>
>> Why does everybody keep insisting this can't be done?
>>
>> http://www.filesystems.org/project-unionfs.html


>What if unionfs is not available?

>How would you make it work then?

You don't.
Or you install unionfs or aufs and run the modules.

On Fri, 09 Oct 2009 11:10:13 -0700, Tony Lawrence (pcunix) wrote :
(...)



Hello guys and thanks for your answers.

Yes, what I'm looking for is a cumulative mount, as you call it
"unionfs".

For instance :

/dev/md4 and /dev/md5 will be moutned as /mnt/data.

In this mount point, I will have a global directory, say GD. On each md
device, I have this GD directory.

Then, on one md device, I have for example, 3 directories, say DA, DB and
DC. On the second md device, I have DD and DE.

my resulting mount point would be then as follows :

/mnt/data/GD/DA/
/mnt/data/GD/DB/
/mnt/data/GD/DC/
/mnt/data/GD/DD/
/mnt/data/GD/DE/

But as you said, my potential problem is that in D[A-E] directories, I
have files with the same name. There is just the main directory (DA, DB,
etc) which differs.

Is that a real problem?

Anyway, I've just learned something. I didn't know the existence of these
2 modules :
- unionfs
- aufs

I will look them with attention.

Thanks

climb65 wrote:
> On Fri, 09 Oct 2009 11:10:13 -0700, Tony Lawrence (pcunix) wrote :
> (...)
>
>
>
> Hello guys and thanks for your answers.
>
> Yes, what I'm looking for is a cumulative mount, as you call it
> "unionfs".
>
> For instance :
>
> /dev/md4 and /dev/md5 will be moutned as /mnt/data.
>
> In this mount point, I will have a global directory, say GD. On each md
> device, I have this GD directory.
>
> Then, on one md device, I have for example, 3 directories, say DA, DB and
> DC. On the second md device, I have DD and DE.
>
> my resulting mount point would be then as follows :
>
> /mnt/data/GD/DA/
> /mnt/data/GD/DB/
> /mnt/data/GD/DC/
> /mnt/data/GD/DD/
> /mnt/data/GD/DE/
>
> But as you said, my potential problem is that in D[A-E] directories, I
> have files with the same name. There is just the main directory (DA, DB,
> etc) which differs.
>
> Is that a real problem?

Also look into LVM for concatenating volumes into one logical volume, RAID0.