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Re: [linux-lvm] Snapshots and disk re-use

On 06/04/2011 00:11, James Hawtin wrote:


That's fantastic! Thanks very much! I have a couple of questions:

1) If I wanted to create a script that backed up lots of customer-data LVs, could I just do one zero at the end (and still have no data leakage)?

Yes you could, because COW means COPY ON WRITE, so the original block is copied onto the COW with the data from the original disk overwritting any data currently on it. Before that point any data on it was not addressable from the snapshot lv (* see my final point)
2) On average, each of my data LVs are 20GB each, and if I were to create a snapshot of 20GB, this would take about 20 mins to erase. If I made the snapshot only 1GB, that means it would be quick to erase at the end (however only 1GB of data could be created on the respect origin, correct?)

You are right you only have to erase the snapshot cow space, which is normally only 10-15% of the whole original disk. 2GB is pretty fast to over right on any system I have used these days. To be sure though you do need to overwrite the whole cow even if only a few percent was used as you cannot tell which few percent that was.
Actually, I meant just making a snapshot of 1GB, not just erase the first 1GB of a 20GB snapshot. Bu tthis may be moot (See below)

* I do wonder why you are so worried, leakage is only a problem if the COW is assigned to a future customer LV. If you always used the same space for backups perhaps had a PE just for backups it would never be used in a customer lv therefore you could argue that you never have to erase it. If its on a pe you only use for snapshotting you also don't need to hog the space as any bit of that disk is ok.
Excellent point! As long as I use the same PEs for making the snapshot everytime, I don't need to ever erase it (And it can be a nice big size like 50GB, so even my largest customers won't outgrow the snapshot). However though, wouldn't I need to keep the "hog" around just to make sure that the snapshot PEs don't get assigned to a new customer LV in the future (Currently, we don't specify PEs to use when creating normal LVs)?

An even better question: Does the snapshot have to be on the same physical disk as the LV its mirroring?

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