What does an LVM Snapshot consist of ?

herecomesbrod

RE: Chapter 23.

I have read several articles about LVM snapshots, but still can't quite understand how they work.

Some sources suggest that the snapshot is an exact, point-in-time copy of a logical volume, however there are also references to the fact that the initial snapshot requires less space than the original volume. How can both be true ? What is missing from the snapshot that makes it smaller ?

luisviveropena

Hi @herecomesbrod ,

The most of times, having a look to the man page of the tool is useful. So, if we do "man lvcreate", this will show the following for "-s":

-s, --snapshot OriginalLogicalVolume{Name|Path}
Create a snapshot logical volume (or snapshot) for an existing, so called original logical volume (or origin). Snapshots provide a 'frozen image' of the contents of the origin while the origin can still be updated. They enable consistent backups and online recovery of removed/overwritten data/files. Thin snapshot is created when the origin is a thin volume and the size is not specified. Thin snapshot shares same blocks within the thin pool volume. The snapshot with the specified size does not need the same amount of storage the origin has. In a typical scenario, 15-20% might be enough. In case the snapshot runs out of storage, use lvextend(8) to grow it. Shrinking a snapshot is supported by lvreduce(8) as well. Run lvdisplay(8) on the snapshot in order to check how much data is allocated to it. Note that a small amount of the space you allocate to the snapshot is used to track the locations of the chunks of data, so you should allocate slightly more space than you actually need and monitor the rate at which the snapshot data is growing so you can avoid running out of space.

For, first thing we know now, is that a LVM snapshot will use (at the beginning at least) 15-20% of the original LVM volume size. And why's that? Because the snapshot works like a pointer to the blocks of the original lvm volume. So, when the original volume changes, the snapshot gets bigger because it needs to backup a bigger part of the original content (it uses a copy-on-write mechanism).

I found useful information here: https://www.clevernetsystems.com/lvm-snapshots-explained/
and here: https://serverfault.com/questions/41020/is-this-how-lvm-snapshots-work

I hope that helps.

Many regards,
Luis.

herecomesbrod

Thanks for your help Luis. I understand now. The Clever Net Systems reference is a very good explanation.

luisviveropena

It's a pleasure!

Many regards,
Luis.