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[[TableOfContents]] <<TableOfContents>>

LVM

The aim of this page is to summarize the LVM relevant parts.

Contents

  1. LVM
    1. Prerequisites
    2. Preparations
    3. make LVM snapshot
    4. restore LVM snapshot time
    5. turnaround time

Prerequisites

Installed and running XEN Dom0. (with NAT)

In the test a Debian sid netinstall was used.

Preparations

  • (1) Test if module is installed: 

test:~# modprobe dm-mod
test:~# lsmod|grep dm
dm_snapshot            15552  0
dm_mirror              19152  0
dm_mod                 50232  2 dm_snapshot,dm_mirror

(2) Install Packages 

aptitude install lvm2
=>  lvm-common lvm2

(3) Adjust Drive Range 

vim /etc/lvm/lvm.conf
 
    # By default we accept every block device
    # filter = [ "a/.*/" ]
    filter = [ "a|/dev/sda4|", "r/.*/" ]

(4) Make sure we have no pre-installed drives 

vgscan
  Reading all physical volumes.  This may take a while...
  No volume groups found
vgchange -a y
  No volume groups found

(5) Create a Volume 

pvcreate /dev/sda4
  Physical volume "/dev/sda4" successfully created

(6) Create Volume Group 

vgcreate vg /dev/sda4
  Volume group "vg" successfully created
(not done: vgextend vg /dev/sdb4)

(7) Create virtual Machine 

xen-create-image --hostname buildsid --ip 10.0.2.40 --gateway 10.0.2.40

make LVM snapshot

lvcreate -L 2G --snapshot -n buildsid-disk-ss /dev/vg/buildsid-disk

restore LVM snapshot time

The aim of this section is to measure the time

(1) Start virtual machine

(2) Log into machine

(3) Make some changes 

buildsid:~# dd bs=1024 count=100000 if=/dev/urandom of=test.img
100000+0 records in
100000+0 records out
102400000 bytes (102 MB) copied, 25.68 s, 4.0 MB/s

(4) This makes some changes to the snapshot: 

  lvdisplay /dev/vg/buildsid-disk-ss
  [...]
  LV Size                10,00 GB
  Current LE             2560
  COW-table size         2,00 GB
  COW-table LE           512
  Allocated to snapshot  4,86%
  Snapshot chunk size    8,00 KB
  [...]

(5) Shut down the machine

(6) An now the test: 

time lvremove -f /dev/vg/buildsid-disk-ss; time lvcreate -L 2G --snapshot -n buildsid-disk-ss /dev/vg/buildsid-disk

(7) The result is: 

  Logical volume "buildsid-disk-ss" successfully removed

real    0m0.088s
user    0m0.000s
sys     0m0.036s
  Logical volume "buildsid-disk-ss" created

real    0m0.225s
user    0m0.000s
sys     0m0.048s

(8) A second run on an unchanged snapshot: 

  Logical volume "buildsid-disk-ss" successfully removed

real    0m0.067s
user    0m0.000s
sys     0m0.032s
  Logical volume "buildsid-disk-ss" created

real    0m0.170s
user    0m0.000s
sys     0m0.052s

Subjective observation: Sometimes it can take up to 3 seconds.

(9) Results:

remove

create

total time

Debian Sit 10GB/2GB

0.088s

0.225s

0.243s

turnaround time

The trunaround time is the time from t0 (shutting down machine A) to tn (machine B is up). The following stages can be forseen:

Time

Event

t0

trigger shutdown

t1

shutting down machine A

t2

machine is down

t3

start delete snapshot

t4

end delete snapshot

t5

start create snapshot

t6

end create snapshot

t7

start booting machine B

t8

end of booting machine B

t9

sending signal from B

Preparations:

(1) boot the machine

(2) make it is up and running

(3) Copy a ssh key for a user on that

  • machine to the autorized_key file, so that the user can log in to that machine without password.

The following script measures the time: 

time turnaround.pl

The "turnaround.pl" script do work 

TODO