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$Date: 2002/01/22 14:49:34 $
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Logical Volume Manager Abstract
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Goals:
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------
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Implement a very flexible virtual disk subsystem to handle disk storage.
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Online allocation and relocation of storage.
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Online extension and reduction of storage.
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Function:
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---------
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The Logical Volume Manager (LVM) adds an additional layer between
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the physical peripherals and the i/o interface in the kernel to
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get a logical view of disks.
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This allows the concatenation of several disks (so-called physical volumes
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or PVs) to form a storage pool (so-called Volume Group or VG) with
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allocation units called physical extents (called PE).
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Parts out of this VG then can be allocated in form of
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so-called Logical Volumes or LVs in units called logical extents or LEs.
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Each logical extent is mapped to a corresponding physical extent
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of equal size. These physical extents are mapped to offsets and blocks
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on the disk(s).
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The LVs can then be used through device special files similar to
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/dev/sd[a-z]* or /dev/hd[a-z]* named /dev/VolumeGroupName/LogicalVolumeName.
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But going beyond this, you are able to extend or reduce VGs AND LVs at runtime.
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Concept:
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--------
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The configuration information for the physical volume, volume group and
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logical volume(s) is stored on each physical volume and in automatically
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created backup files, which are stored in the /etc/lvmtab.d directory.
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The config area on the disk(s) is called Volume Group Descriptor Area or VGDA.
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A LVM driver holds mapping tables between the LEs of LVs and the PEs of PVs.
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These tables are created/updated/deleted by superuser LVM commands.
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The main mapping function of the driver is called with a logical block
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in a LV from functions in /usr/src/linux/drivers/block/ll_rw_blk.c
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(in functions ll_rw_block() and ll_rw_swap_file() ) and looks up the
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corresponding physical block/disk pair in a table.
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Then it returns this pair to the calling ll_rw_*() function causing a
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physical i/o request to the disk block(s) to be queued.
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Example:
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--------
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If the capacity of a LV gets too small and your VG containing this LV is full,
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you could add another PV to that VG and simply extend the LV afterwards.
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If you reduce or delete a LV you can use the freed capacity for different
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LVs of the same VG.
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The above scenario looks like this:
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/------------------------------------------\
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| /--------\ VG 1 /--------\ |
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| | | | | |
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| | PV 1 | ...... | PV n | |
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| | /-----------------------\ | |
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| | \-------LV 1------------/ | |
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| \--------/ \--------/ |
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\------------------------------------------/
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PV 1 could be /dev/sdc1
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PV n could be /dev/sde1
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VG 1 could be vg00
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LV 1 could be /dev/test_vg/test_lv
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The configuration steps for getting the above scenario are:
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1. after installing LVM do an "insmod lvm" or
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setup kerneld/kmod to load it automatic (see INSTALL)
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2. set up partitions (#1) on both disks with partition type 0x8e
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(I used this type to avoid using Linux primary partitions etc.)
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3. do a "pvcreate /dev/sd[ce]1"
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For testing purposes you can use more than one
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primary and/or extended partition on a disk.
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Don't do that for normal LVM operation for performance reasons.
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Unless you have to, don't stripe logical volumes over physical volumes
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associated to partitions on the same disk.
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4. do a "vgcreate test_vg /dev/sd[ce]1"
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(vgcreate activates the volume group too)
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5. do a "lvcreate -L100 -ntest_lv test_vg" to get a 100MB linear LV
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or a "lvcreate -i2 -I4 -l100 -nanother_test_lv test_vg" to get a
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100 LE large logical volume with 2 stripes and stripe size 4 KB.
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6. use created LVs as you want to.
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For example generate a filesystem in one with "mke2fs /dev/test_vg/test_lv"
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and mount it.
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Overview and concept of commands:
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---------------------------------
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I grouped and named LVM commands analogous to HP's.
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So the commands for physical volume handling all start with pv,
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those for volume group handling start with vg and the ones for
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logical volumes start with lv.
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e2fsadm - administration wrapper for logical volume including filesystem
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resizing for lvextend, lvreduce, e2fsck and resize2fs
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lvchange - change attributes of a logical volume
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lvcreate - create a logical volume
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lvdisplay - display logical volume config data
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lvextend - extend a logical volume in size
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lvreduce - reduce a logical volume in size
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lvremove - remove a logical volume
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lvrename - rename an inactive logical volume
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lvscan - find all existing logical volumes
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lvmchange - emergency program to change attributes of the LVM
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lvmdiskscan - scan all disks / partitions and multiple devices and list them
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lvmsadc - statistic data collector
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lvmsar - statistic data reporter
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pvchange - change attributes of physical volumes
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pvcreate - create a physical volume
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pvdata - debug list physical volume group descriptor area
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pvdisplay - display physical volume config information
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pvmove - move logical extents to a different physical volume
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pvscan - find all existing physical volumes
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vgcfgbackup - backup all volume group descriptor areas
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vgcfgrestore - restore volume group descriptor area(s) to disk(s)
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vgchange - activate/deactivate volume group(s)
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vgck - check volume group descriptor area for consistency
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vgcreate - create a volume group from physical volume(s)
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vgdisplay - display volume group config information
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vgexport - export volume group (make it unknown to the system)
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vgextend - extend a volume group by one or more physical volumes
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vgimport - import a volume group (make it known to the/another system)
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vgmerge - merge two volume groups into one
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vgmknodes - create volume group directory with all logical volume specials
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vgreduce - reduce a volume group by one or more empty physical volume(s)
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vgremove - remove an empty volume group
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vgrename - rename an inactive volume group
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vgscan - scan for volume groups
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vgsplit - split one volume group into two
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Example LVM session output:
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---------------------------
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# create physical volumes on 9 SCSI disk primary partition 1
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pvcreate /dev/sd[b-eg-k]1
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pvcreate -- /dev/sdb1 has an invalid physical volume identifier
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pvcreate -- physical volume on /dev/sdb1 successfully created
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pvcreate -- reinitializing physical volume
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pvcreate -- physical volume on /dev/sdc1 successfully created
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pvcreate -- reinitializing physical volume
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pvcreate -- physical volume on /dev/sdd1 successfully created
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pvcreate -- reinitializing physical volume
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pvcreate -- physical volume on /dev/sde1 successfully created
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pvcreate -- reinitializing physical volume
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pvcreate -- physical volume on /dev/sdg1 successfully created
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pvcreate -- reinitializing physical volume
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pvcreate -- physical volume on /dev/sdh1 successfully created
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pvcreate -- reinitializing physical volume
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pvcreate -- physical volume on /dev/sdi1 successfully created
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pvcreate -- reinitializing physical volume
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pvcreate -- physical volume on /dev/sdj1 successfully created
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pvcreate -- reinitializing physical volume
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pvcreate -- physical volume on /dev/sdk1 successfully created
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# create a volume group with default physical extent size
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# from these physical volumes
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vgcreate my_first_vg /dev/sd[b-eg-k]1
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vgcreate -- INFO: using default physical extent size 4096 KB
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vgcreate -- INFO: maximum logical volume size is 256 GB
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vgcreate -- doing automatic backup of vg05
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vgcreate -- volume group my_first_vg successfully created
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# Oops ;-)
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# Don't like the limitations caused by default physical extent size
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# --> deactivate and delete volume
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vgchange -an my_first_vg
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vgchange -- my_first_vg successfully deactivated
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vgremove my_first_vg
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vgremove -- volume group my_first_vg successfully removed
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# create a volume group with physical extent size of 8192 KB
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# from these physical volumes
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vgcreate -s 8192 my_first_vg /dev/sd[b-eg-k]1
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vgcreate -- INFO: maximum logical volume size is 512 GB
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vgcreate -- doing automatic backup of my_first_vg
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vgcreate -- volume group my_first_vg successfully created
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# display volume group config
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vgdisplay my_first_vg
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--- Volume group ---
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VG Name my_first_vg
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VG Write Access read/write
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VG Status available
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VG # 1
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MAX LV 31
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Cur LV 0
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Open LV 0
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MAX LV Size 512 GB
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Max PV 256
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Cur PV 9
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Act PV 9
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VG Size 12636 MB
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PE Size 8192 KB
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Total PE 1579
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Alloc PE / Size 0 / 0 KB
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Free PE / Size 1579 / 12632 MB
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# do it again Sam but verbose
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vgdisplay -v my_first_vg
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--- Volume group ---
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VG Name my_first_vg
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VG Write Access read/write
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VG Status available
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VG # 1
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MAX LV 31
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Cur LV 0
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Open LV 0
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MAX LV Size 512 GB
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Max PV 256
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Cur PV 9
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Act PV 9
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VG Size 12636 MB
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PE Size 8192 KB
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Total PE 1579
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Alloc PE / Size 0 / 0 KB
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Free PE / Size 1579 / 12636 MB
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--- No logical volumes defined in my_first_vg ---
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--- Physical volumes ---
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PV Name (#) /dev/sdb1 (1)
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PV Status available / allocatable
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Total PE / Free PE 131 / 131
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PV Name (#) /dev/sdc1 (2)
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PV Status available / allocatable
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Total PE / Free PE 131 / 131
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PV Name (#) /dev/sdd1 (3)
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PV Status available / allocatable
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Total PE / Free PE 131 / 131
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PV Name (#) /dev/sde1 (4)
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PV Status available / allocatable
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Total PE / Free PE 131 / 131
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PV Name (#) /dev/sdg1 (5)
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PV Status available / allocatable
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Total PE / Free PE 131 / 131
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PV Name (#) /dev/sdh1 (6)
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PV Status available / allocatable
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Total PE / Free PE 131 / 131
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PV Name (#) /dev/sdi1 (7)
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PV Status available / allocatable
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Total PE / Free PE 131 / 131
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PV Name (#) /dev/sdj1 (8)
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PV Status available / allocatable
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Total PE / Free PE 125 / 125
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PV Name (#) /dev/sdk1 (9)
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PV Status available / allocatable
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Total PE / Free PE 537 / 537
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# create a linear physical volume with all space of the volume group
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lvcreate -l1579 my_first_vg
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lvcreate -- doing automatic backup of my_first_vg
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lvcreate -- logical volume /dev/my_first_vg/lvol1 successfully created
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# create an ext2fs on newly created logical volume
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mke2fs /dev/my_first_vg/lvol1
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mke2fs 1.10, 24-Apr-97 for EXT2 FS 0.5b, 95/08/09
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Linux ext2 filesystem format
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Filesystem label=
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3235840 inodes, 12939264 blocks
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646963 blocks (5.00%) reserved for the super user
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First data block=1
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Block size=1024 (log=0)
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Fragment size=1024 (log=0)
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1580 block groups
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8192 blocks per group, 8192 fragments per group
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2048 inodes per group
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Superblock backups stored on blocks:
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8193, 16385, 24577, 32769, 40961, 49153, 57345, 65537, 73729,
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81921, 90113, 98305, 106497, 114689, 122881, 131073, 139265, 147457,
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<SNIP>
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Writing superblocks and filesystem accounting information: done
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0.66user 26.55system 3:06.62elapsed 14%CPU (0avgtext+0avgdata 0maxresident)k
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0inputs+0outputs (97major+522minor)pagefaults 0swaps
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# and mount it (not very exiting :-) )
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mount /dev/my_first_vg/lvol1 /mnt
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