Text 655, 56 rader
Skriven 2006-07-31 12:09:00 av Angus McLeod (369.dove-nix)
    Kommentar till text 654 av Finnigann (368.dove-nix)
Ärende: Re: I *love* LVM!
=========================
  Re: Re: I *love* LVM!
  By: Finnigann to Angus McLeod on Mon Jul 31 2006 06:24:00

 >  Well, sorta like Partician Magic on steroids.

I've not used Partition Magic, but it is similar in some respects.  What I 
do not know is if Partition Magic can combine Physical Volumes before it 
begins partitioning.  What LVM does is this:

      +-------+      +-------+      +-------+   These are the Physical
      |  hda  |      |  hdb  |      |  hdc  |   Volumes (actual IDE, 
      | 40 Gb |      | 60 Gb |      | 80 Gb |   SATA, SCSI drives) that 
      +-------+      +-------+      +-------+   are attached to the system
              \          |          /        
               \         |         / 
             +-----------------------+          The VG is a single block
             | Volume Group:  180 Gb |          containing all the space
             +-----------------------+          on all the PVs.
                /        |        \
               /         |         \
      +-------+      +-------+      +-------+   Logical Volumes allocated
      |  LV1  |      |  LV2  |      |  LV3  |   out of the space in the VG
      | 90 Gb |      | 50 Gb |      | 15 Gb |   look exactly like real hard
      +-------+      +-------+      +-------+   drives to the OS and user.

In this example there is 25 Gb space "free" in the VG.  If more space is 
needed in any of the LVs, they can be expanded by allocating them some of 
this 25 Gb.  Unwanted space in a LV can be deallocated back to the VG and 
then reallocated as needed to another LV.  If you need more extra space 
than your VG contains, you can buy a new drive, add it as a PV to the VG, 
and then allocate that space to the LVs as required.  Very sweet.

Another thing that LVM can do is create 'snapshots' of logical volumes.  
A snapshot is a read-only copy of another volume that contains all the 
data that was in the volume at the time the snapshot was created.  This 
means we can process that volume without having to worry about data being 
changed while we are processing, and we don't have to take the volume 
offline while the process is taking place.  Say we want to do a lengthy 
month-end analysis or a daily backup.  We don't want the data changing 
while we do the process, but we don't want our system offline for any 
length of time either.  We make a snapshot of the LV (which only takes a 
couple of seconds and occupies *far* less space than the original LV), and 
while the original LV is still in use, we process the snapshot.  When our 
processing is complete, we delete the snapshot.

I've not tried using snapshots because I have no need for them at present.  
In the past I could have *really* used them, but they were only available 
as a part of the VERITAS Extent-based File System, the OS vendor wantes a 
PILE of cash for the reature, and The Bosses refused to pay.

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