Re: [linux-lvm] Sanity check: newbie wants strange LVM configuration

[Chris Worley <cworley@liberate.com>]

Joe,

Good plan, but I have some questions...

Joe Thornber wrote:

> 6) turn the 12G drive into a PV
> 7) extend the volume group with the new PV
> 8) create a striped logical volume big enough to hold the 12G

And LVM knows automagically to grab 12GB from the 40GB drive to stripe 
together into a 24GB drive?


> OK so you should now have (40 - 6)G free on the big disk, and (12 -6)G 
> free on the smallest disk.

Why don't I have 40-12 and 12-12 (nothing free on the smaller disk) 
resulting in a 24G raid0?

Also, I think it would be better to do the 16G first, then the 12G. 
Is there any reason why you did the 12G first, or is this interchangeable?


> 16) create a 16G striped LV

Shouldn't this be a 32G striped LV (16G from the 16G drive, another 
16G from the 40G drive)?


> You now have a striped LV 12G, a striped LV 16G + lots of left over
> extents that can be added when needed.

Wouldn't that be a 24G striped LV and a 32G striped LV that I can 
append together into a 56G LV (and lots of non-striped extents that I 
can add to the end of that)?

Thanks for your help,

Chris

> On Tue, Jan 02, 2001 at 10:22:34PM -0700, Chris Worley wrote:
> 
>> I've got two disk drives, 12 & 16 GB IDE.  I'm adding a third 40GB
>> drive.  Time to start using LVM...
>> 
>> I want to get better performance, so, I'll put the 40GB drive on one
>> IDE controller, the other two drives on the second controller.  I'll
>> make two partitions on the 40GB drive that match the disk sizes of the
>> existing drives, and stripe each with it's match on the 40GB drive,
>> creating two raid0 arrays (one 12+12, and the other 16+16).  By
>> placing the dual-partitioned 40GB drive stand-alone on one IDE
>> controller, each striped with a partition/drive on the other
>> controller, any given file will only be striped across one partition
>> on each controller, so I should see the performance benefit of
>> striping (on IDE).
>> 
>> Should I use LVM or MD to do the striping (they both can do it, I was
>> just wondering which would be a better choice)?
>> 
>> Even if I use MD to stripe, I'd use LVM to append the two drives
>> together.
>> 
>> Before appending the drives, I'd make a temporary partition on the
>> 40GB drive, and copy the current contents of the 16GB (/home) drive to
>> the temporary partition.  Then, I'd make the 16+16 raid0 a logical
>> volume, create a reiserfs on it, and copy the information back from
>> the temporary partition, to the new reiserfs.
>> 
>> Since the 12GB drive is the current root partition, it's a bit
>> trickier to copy.  I'd copy it's contents to a temporary partition on
>> the 40GB drive, boot from that temporary partition, then create the
>> second 12+12 raid0, and add it to the first logical volume, then
>> expand the reiserfs to cover both, copy the root file system from the
>> temporary partition to the new logical volume, and setup a reiserfs
>> root and boot.
>> 
>> Is this the correct approach for upgrading?
>> 
>> Finally, I'll have ~10GB unallocated on the 40GB drive.  I was
>> thinking of adding this to the end of the current logical volume (and,
>> again, expand the reiserfs to cover the additional space).
>> 
>> Since any file system looses performance when more than 90% full, this
>> final non-striped partition would be in a position where performance
>> would degrade anyway, and keep the raid0's in a position for full
>> performance.
>> 
>> Is that correct?
>> 
>> Sort of off-topic (not LVM related)...
>> 
>> I've got an IDE CDROM drive that I want to put on the same controller
>> as the 40GB drive.  I've been told that new UDMA drives do not have
>> the PIO performance hit associated with CDROM drives, so I should be
>> able to get full performance from my 40GB drive, even with a CDROM on
>> the same IDE controller.
>> 
>> Is that correct (or should I junk the IDE CDROM)?
>> 
>> Thanks,
>> 
>> Chris