LVM 1.0.8
						$Date: 2003/11/17 15:58:54 $
Logical Volume Manager frequently asked questions:               
--------------------------------------------------

Q0: Where can I find more information about the
    Logical Volume Manager for Linux?

A0: Please see <http://www.sistina.com/lvm> and/or join
    our mailing list at <Linux-LVM@sistina.com> by following the subscription
    instructions at <http://www.sistina.com/lvm>.
    You can find pointers to LVM howtos on the Web there as well.



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 99 volume groups at the moment.
    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 0x8e 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 is limited
    by the size of the physical extents times their maximum amount, 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 implicitly 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 slowly,
    what's going on?

A5: If you put it on two or more physical volumes based on partitions on
    one disk, you will not gain any 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 can I not 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: If this relly is the case after following the instructions
    contained in PATCHES/README please send an email to <linux-lvm@sistina.com>.


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 NOT an 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
    32 bit kernels are limited to 2 Terabytes (with 512 byte block size).



Q10: Why can't I split my volume group my_vg?

A10: The physical volumes you want to split into another volume group
     may NOT have logical extents of logical volumes belonging to 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 (currently) 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@Sistina.com> or
     to the linux-lvm mailing list.


Q14: Where can I ask for a missing feature or for a better way to implement
     something in the LVM?

A14: Please send me an enhancement mail request
     to <Linux-LVM-Enhancement@Sistina.com> or
     to the linux-lvm mailing list.


Q15: Why is LVM not in the stock kernel?

A15: LVM has been in the stock kernel since 2.3.47


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@Sistina.com>


Q17: Why am I not able to create my 211th logical volume?
     vgdisplay tells me about a limit of 256 logical volumes.

A17: The total amount of 256 logical volumes is shared among all
     volume groups. You have to delete logical volumes in different
     volume groups to be able to create the new one.


Q18: Can I have my root filesystem in a logical volume?

A18: Yes you can. There's basic support since LVM 0.7 to create an initial RAM
     disk containing the necessary executables, device specials etc.
     to switch to a logical volume containing e root filesystem.
     See script lvmcreate_initrd(8).
     Nevertheless you have to setup the neccesary logical volumes with
     filesystems, /etc/lilo.conf and /etc/fstab by hand.

Q19: Why does LVM not support mirroring?

A19: Mirroring or other levels of RAID on a per Logical Volume
     base decreases reliability because you have to take care of
     which Logical Volumes are mirrored and which ones are not.
     I recommend to use dedicated hardware RAID subsystems or Multiple
     Devices to have the redundancy below the LVM. In this case you
     just don't care about Logical Volume data redundancy and you don't
     run into the dangerous situation that your data is not redundant
     by accident.
     If you do have these kind of devices in place you can setup RAID 10/40/50
     if you want to because the LVM supports RAID0. Combining for eg. several
     hardware RAID5 subsystems in a volume group to set up LVM-RAID0 gives you
     reliability, performance and flexibility at the same time.