Great ! Your explanation is perfect :o) So, I understand better the use of a snapshot. The /data contains files sharing with Samba. So, I think I can do one snapshot per day at midnight, and remove it immediatly. If I have to restore data for the snapshot, its's alway possible to mount the /dev/OVG/snap_admin ? Thanks in advance and happy new year ! David Johnston a écrit : > On Wed, 2003-12-31 at 02:17, Jean Marie Ariès wrote: > > Hi, > > > > I'm using LVM under SuSE Linux 8.0, and I don't understand all the > > snapshot process. > > > > Our config is : > > 2 disks 73 Go RAID 1 > > --> 1 VG "OVG" (60 GB) > > --> 1 LV /dev/OVG/data (40 GB) mounted on /share/data > > > > To create a snapshot with ReiserFS, we do this: > > Tux :# lvcreate -L30G -s -n snap_admin /dev/OVG/data > > and so, we mount the snapshot : > > Tux :# mount /dev/OVG/snap_admin /share/snapshot > > > > At this point, the /share/snapshot contain a copy of the /share/data > > directory. > > > > The How To say "Do the backup" for exemple with tar. > > > > My question is : Why we must to copy the data with a system command on > > the snapshot ? Is a "create snapshot/remove snapshot" not possible ? > > Jean Marie, > The snapshot is useful for narrowing a backup window, but is not useful > as a backup. > > In other words, a backup can take hours while a snapshot takes seconds. > This is possible because the snapshot creation process does not copy > your data; it simply sets up a process that will track any future > changes. > > Once you've created a snapshot, you can go back to work (restart your > database, for example). The data in the original LV (/dev/OVG/data) > will change, but the data in /share/snapshot won't. Once you've copied > /share/snapshot/ somewhere safe, you remove the snapshot with lvremove. > The How-To suggests tar, but other commands are possible. > > Why not leave the snapshot in place all the time? Well, there are three > reasons. > > 1) As long as the snapshot exists, writes to /dev/OVG/data will be > slower than usual because they have to be done twice (once to record the > new data in /dev/OVG/data, once to record the changes in > /dev/OVG/snap_admin). > > 2)If you leave a snapshot in place long enough, it will run out of space > to store the changes; once this happens, your snapshot is useless. In > the example you gave, you can make roughly 30GB worth of changes before > the snapshot fills up. You can extend the snapshot to the size of the > original lv (40GB) but not beyond. > > 3) If the disk drive fails, both the original LV and the snapshot will > be lost. > > I hope my explanation is clear and correct. If not, somebody please let > me know ;-) > -- > David Johnston <david@littlebald.com> > Little Bald Consulting, LLC > > _______________________________________________ > linux-lvm mailing list > linux-lvm@sistina.com > http://lists.sistina.com/mailman/listinfo/linux-lvm > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/ -- Sincèrement, Jean Marie Ariès Imprimeries IPS Responsable Systèmes/Réseaux _______________________________________________ linux-lvm mailing list linux-lvm@sistina.com http://lists.sistina.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
