Logical Volumes

[David A Higgen]

In article <1990Oct22.171615.17166 at cid.aes.doe.CA>, aspgasd at cidsv01.cid.aes.doe.CA (Alain St-Denis) writes:
> Is there a manual that explains all there is to know about logical
> volumes? I already read the System Administrator's Guide. 

Suffering from a touch of multiple-post-itas, aren't we Alain? As I said
in my last follow-up, I will try to answer specific questions.

> I would like to know how the stripping is implemented and also how the
> file space is allocated on a logical volume extended with growfs.
			       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

No, no: this seems to be a widespread confusion. Growfs extends the
*filesystem*, not the logical volume. A logical volume is extended by
adding more devices to its specification in /etc/lvtab and rerunning
mklv; you would do this *before* using growfs.

I think there is some confusion because many people have heard the 
terms "striped files" or "striped filesystems".

There are basically two ways you can do volume management & software striping.

1)  You can modify the filesystem to have knowledge of the physical devices
    on which it resides, and add management policies to the filesystem 
    itself to allow it to determine which physical disk a given file block
    is on,

OR

2)  You can build an intermediate layer, below the filesystem but above the
    actual disk drivers, which makes multiple real disk devices look to
    higher layers like a single device.

We have used the latter approach: it is more modular and allows volumes to
be constructed from any type of physical disk without modifications to the
filesystem code. Our logical volumes are *devices*, the filesystem knows
nothing about them.

			Dave Higgen (daveh at xtenk.asd.sgi.com)