Snapshot Creation

Creating a logical volume (LV) snapshot is much the same process as creating a LV (see our LVM Configuration KB for details on how to create a LV.)  However, when creating a LV snapshot you must use a -s command line switch in conjunction with the LV you wish to snapshot.  The syntax is as follows:

• lvcreate -L <SIZE_OF_SNAPSHOT> -s -n <NAME_OF_SNAPSHOT> <LV_TO_SNAPSHOT>

In the following example, we will create a 500MB snapshot LV of an existing LV, lets being:

1. Use lvdisplay to find the name of the logical volume you wish to snapshot
   
   

   [root@Linux01 ~]# lvdisplay --- Logical volume --- LV Name /dev/TCPDumpVolGRP/TCPDumpLV VG Name TCPDumpVolGRP LV UUID hYQs4t-YtY7-51hl-c4ps-4N6d-2W7h-IidcxF LV Write Access read/write LV Status available # open 1 LV Size 1.50 GB Current LE 48 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 253:5 ... OUTPUT TRUNCATED

   
   
2. Create a new 500MB snapshot from the source LV /dev/TCPDumpVolGRP/TCPDumpLV
   
   

   [root@Linux01 /]# lvcreate -L 500M -s -n BackupLV /dev/TCPDumpVolGRP/TCPDumpLV Rounding up size to full physical extent 512.00 MB Logical volume "BackupLV" created [root@Linux01 ~]#

   Note: Our snapshot LV (500MB) is about 1/3 the size of the source LV (1.5 GB).  As we have already said, the snapshot LV does not need to be the same size of the source.  Because the snapshot LV will only contain the changes made to the source LV while its snapshot, and we know there is not a high rate of change on the source LV, we are fine using 1/3 the size (and would have probably been safe making it far less.)
   
   
3. Verify the snapshot has been created
   
   

   [root@Linux01 ~]# lvdisplay --- Logical volume --- LV Name /dev/TCPDumpVolGRP/TCPDumpLV VG Name TCPDumpVolGRP LV UUID hYQs4t-YtY7-51hl-c4ps-4N6d-2W7h-IidcxF LV Write Access read/write LV Status available # open 1 LV Size 1.50 GB Current LE 48 Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 253:5 --- Logical volume --- LV Name /dev/TCPDumpVolGRP/BackupLV VG Name TCPDumpVolGRP LV UUID mSWMF0-5JtO-GkAd-plBb-YIf8-1HOg-JRfV34 LV Write Access read/write LV snapshot status active destination for /dev/TCPDumpVolGRP/TCPDumpLV LV Status available # open 0 LV Size 1.50 GB Current LE 48 COW-table size 512.00 MB COW-table LE 16 Allocated to snapshot 0.00% Snapshot chunk size 4.00 KB Segments 1 Allocation inherit Read ahead sectors auto - currently set to 256 Block device 253:7 ... OUTPUT TRUNCATED

   Note: You will notice on the BackupLV that although the LV size says that it's 1.5GB, the copy-on-write (COW) table informs us that it's actually only 512MB (the source LV is 1.5GB.)  Also, the percentage allocated to the snapshot is currently at 0%.  As updates are made to the source LV, you'll notice this percentage will increase.
   
   
4. We can now mount the snapshot so that it may be backed-up
   
   

   [root@Linux01 /]# mkdir -p /mnt/backup [root@Linux01 /]# mount /dev/TCPDumpVolGRP/BackupLV /mnt/backup/ [root@Linux01 /]# df -kh Filesystem Size Used Avail Use% Mounted on /dev/mapper/VolGroup00-LogVol04 3.9G 2.2G 1.6G 59% / /dev/sda1 99M 12M 82M 13% /boot tmpfs 1006M 0 1006M 0% /dev/shm /dev/mapper/VolGroup00-LogVol00 992M 41M 901M 5% /home /dev/mapper/VolGroup00-LogVol02 992M 69M 872M 8% /tmp /dev/mapper/VolGroup00-LogVol03 2.0G 150M 1.7G 8% /var /dev/mapper/TCPDumpVolGRP-4GLV 4.0G 137M 3.7G 4% /4GLV /dev/mapper/TCPDumpVolGRP-TCPDumpLV 1.5G 920M 497M 65% /TCPDumpLV /dev/mapper/TCPDumpVolGRP-BackupLV 1.5G 920M 497M 65% /mnt/backup

   
   
5. Once the snapshot has been backed-up, unmount the LV and remove it
   
   

   [root@Linux01 /]# umount /mnt/backup/ [root@Linux01 /]# lvremove /dev/mapper/TCPDumpVolGRP-BackupLV Do you really want to remove active logical volume "BackupLV"? [y/n]: y Logical volume "BackupLV" successfully removed [root@Linux01 /]#

Nice work, you now know how-to snapshot a LV.  Good luck!

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hamed  - to appreciate |02/09/2009 07:50:58 thanx for your nice tutorial Reply | Quote

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