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Ammad 01-03-2009 09:20 PM

LVM snapshot between different Groups
 
HI,

Can i create snapshot of lvm on different group,

Code:


e.g data is in /dev/VG01/data and i want to take snapshot on

/dev/VG02/snap1


Is it possible to create this ?

thanks.

acid_kewpie 01-04-2009 02:27 AM

No, that's not possible, as a snapshot is not new disk space, but existing areas of that LV which are not modified. Instead all new data is put onto space with the VG, and all the while the size of changes between the snapshot and the current data is less than the size allocated to the snapshot, it will be valid. You can take a small snapshot and then dd it to a seperate location if that's what you want to achieve.

Ammad 01-04-2009 03:32 AM

i want to backup mysql database, which is 500GB, after making snapshot i want to backup it on Tape.

since i have another volume group (VG4) that have 1.8TB space so i want to snapshot on that.


another question is, while creating snapshot how much space i have to define for 500GB database ?

thanks.

acid_kewpie 01-04-2009 08:02 AM

snapshot space needs to be reserved within each VG, as above, you can't snap to somewhere else, as there is no data actually written anywhere. the point of a snapshot is to freeze a copy in time, so you would, assuming your tape is fast enough, write directly from the snap to the tape. you do not need intermediary disk space if you can write to tape before the snap invalidates itself, which for a database, depends entirely on how it's used during that time. I think you'd do well to do some formal study of LVM and what technologies actually back up the concept of an LVM snapshot, and it should fall into place much better.


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