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Trying to undstand lvm snapshots in linux

Posted on 2014-08-17
Medium Priority
Last Modified: 2014-08-19
I am trying to understand lvm snapshots in rehat 6.0

I just created a lvm snapshot that was half the size of the volume I created the snapshot of.  Only 1/3 of the snapshot I created was used.  

How can a snapshot that is 1/6 the size of the volume I created the snapshot of be of any use ?
Question by:TIMFOX123
LVL 62

Accepted Solution

gheist earned 2000 total points
ID: 40266489
Snapshot is filled with old versions of changed blocks. The longer it sits the bigger it grows.
LVL 25

Expert Comment

by:Zephyr ICT
ID: 40266499
How much data is on the volume you're snapshotting? is it also roughly 1/3 of the volume it resides on?
LVL 13

Expert Comment

ID: 40269654
A snapshot volume is a special type of volume that presents all the data that was in the volume at the time the snapshot was created. This means we can back up that volume without having to worry about data being changed while the backup is going on, and we don't have to take the database volume offline while the backup is taking place.

To create snapshot

# lvcreate -L592M -s -n mybackup /dev/ora/databases
lvcreate -- WARNING: the snapshot must be disabled if it gets full
lvcreate -- INFO: using default snapshot chunk size of 64 KB for "/dev/ora/mybackup"
lvcreate -- doing automatic backup of "ora"
lvcreate -- logical volume "/dev/ora/mybackup" successfully created

To Mount

# mkdir /mnt/ora
# mount /dev/ora/mybackup /mnt/ora
mount: block device /dev/ora/mybackup is write-protected, mounting read-only


Author Closing Comment

ID: 40269954
very good answer

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