Logical Volume Manager frequently asked questions:                   13/02/1999
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Q0: where can i find more information about the Logical Volume Manager
    for Linux?

A0: please have a look at <http://linux.msede.com/lvm> and/or join
    our mailing list at <linux-lvm@msede.com> by "subscribe linux-lvm"
    to <majordomo@msede.com>


Q1: why can't i create my volume group "mygroup"?

A1: you can't create a volume group with the same name as an existing one.
    You can't create more than 8 volume groups at this point in time.
    You are only able to use inititalized physical volumes (see pvcreate(8)).



Q2: why  can't i create a physical volume with pvcreate(8)?

A2: maybe pvcreate(8) complains that the physical volume has already
    been initialized. So... think about "pvcreate -f[f] ...".
    But DON'T DO THAT, if the physical volume belongs to another volume group!
    Remember to set the partition system id to 0xfe with fdisk(8) before
    trying pvcreate(8) on it.



Q3: why am i not able to extend a logical volume?

A3: your volume group is full or you've already reached the maximum
    logical volume size in that volume group. Logical volume size depends
    on the size of the physical extents, which only can be set at
    volume group creation time.
    The default physical extent size is 4MB which limits ;-) logical
    volumes to a maximum of 256 Gigabyte (see vgcreate(8), vgdisplay(8)).
    If your volume group isn't full or you didn't reach the current
    logical volume size limit, your logical volume may have striped
    or contiguous allocation policy. Have a look at the physical volumes
    with vgdisplay or pvdisplay(8) to figure out, if there are not enough
    free (contiguous) physical extents.



Q4: why can't i move my logical volume(s) away from a physical
    volume with pvmove(8).

A4: look at the free space on all destination disks you want to use
    (or which are implicit used) AND at the attributes of the logical
    volumes to be moved.
    Remember: you can't move a contiguous logical volume when there isn't 
              enough free contiguous space on any destination disk.
              In this case you can think about changing from contiguous
              allocation policy to next free and do the attribute change
              with lvchange(8).
              You can't move a striped logical volume either, if there isn't
              enough space for the complete stripe on any destination
              physical volume.
              You can't move to physical volumes which are NOT allocatable.
              Think about changing this with pvchange(8).



Q5: my striped logical volume works horrible slow.
    What's gooing on?

A5: if you put it on two or more physical volumes based on partitions on
    one disk, you are not able to gain performance.
    Remember: you are allowed to use two or more partitions of one disk as
              physical volumes (this only makes sense for next free
              allocated logical volumes on those physical volumes).
              If you have attached two IDE disks to one adapter, you can't
              get parallel i/o on these two disks.



Q6: why am i not able to rename my volume group / logical volume?

A6: you have to deactivate them before you are allowed to rename them
    (see lvrename(8), vgrename(8)).



Q7: the LVM kernel patch in the LVM distribution is not compatible
    with my Linux version.
    Where's help?

A7: please have a look at <http://linux.msede.com/lvm> for additional
    LVM kernel patches.
    If there is no corresponding one available send me a mail request
    with the subject "LVM kernel patch request" to
    <linux.LVM@ez-darmstadt.telekom.de>


Q8: a LVM command was just working when my system crashed...
    Help please!?

A8: bring your system back online and look at the volume group
    backup files in /etc/lvmconf.
    There's at least one called /etc/lvmconf/VolumeGroupName.conf and
    possible more in the backup history called
    /etc/lvmconf/VolumeGroupName.conf.*.old. 
    You can use these backup files to bring the configuration
    back to the one before the crash (see vgcfgrestore(8)).



Q9: why are my logical volumes limited to 256 GB in size?

A9: this is NO absolute limit but it depends on the physical extent size you
    configured at volume group creation time.
    Please use option -s of the vgcreate command to give a larger physical
    extent size. For example with a physical extent size of 524288 KB (512 MB)
    you are able to map a logical volume of 32 Terabyte. Remember that actual
    kernels are limited to 1 Terabyte.



Q10: why can't i split my volume group my_vg?

A10: the physical volumes you want to split of into another volume group
     may NOT have logical extents of logical volumes staying in the original
     volume group you started with. Please use pvmove to seperate
     the logical volumes.



Q11: why can't i merge my two volume groups my_vg1 and my_vg2?

A11: a merged volume group can't go beyond the physical or logical volume
     limits of the destination volume group. This means for eg. that you
     can't merge my_vg1 with 20 logical volumes and my_vg2 with 30 logical
     volumes getting my_vg1, if my_vg1 has a 31 logical volume limit.
     You are only able to merge (up to now) volume groups with equal
     physical extent sizes.


Q12: how can i move parts of my logical volume with very intensive i/o
     to a different physical volume?

A12: please look at pvmove(8) and use the logical extent syntax to do the job.


Q13: i have a compile problem and/or a runtime problem with the LVM.
     Where to ask for help?

A13: Please send me a bug mail request to
     <linux.LVM.bug@ez-darmstadt.telekom.de>>


Q14: where can i ask for a missing feature or for a better way to implement
     this thing?

A14: please send me an enhancement mail request
     to <linux.LVM.enhancement@ez-darmstadt.telekom.de>


Q15: why is LVM not in the stock kernel?

A15: i hope that it will be in 2.3 8*)


Q16: where can i send a patch for the LVM?

A16: please remember to make a unified diff to the original LVM distribution
     and mail the diff with your comments to
     <linux.LVM.patch@ez-darmstadt.telekom.de>
