Questions[snapshot](server)

I am using VMware ESXi. In our team we use to provide snapshots for long term backup.

Then we faced issues like memory spillover and the server got hang up.

I started reading in VMware knowledgebase articles and everywhere. Everywhere it was recommended not to have snapshots for a long time.

Even VMware advised to keep snapshots for maximum of three days.

But our team kept asking us to have at least two permanent snapshots (till deleting the VM). Sometimes we may use the VM for a year).

1. one snapshot is for fresh machine state. (So when we complete testing an application, we will revert back to fresh state and install another application) (If I did not allow that, I may often need to host the VM.)

2. Next snapshot for keeping the VM in some state (maybe they would have found an issue and keep that state for some time. Or they may install prerequisites for the application and keep the machine ready for testing.)

Logically, their needs seems to be fair. But if I allow that, I am to permit them to hold the snapshots for long time. We are not using our VM as a mail server or database server.

Why is keeping snapshots for long time having an adverse effect?

Why are snapshots considered as temporary backups, not real backups?

As far as I know, LVM makes it possible to take snapshots of a volume. There are also a number of file systems (ZFS, Btrfs, reiserfs, ...) which supports snapshots.

However, I've never understood the difference between LVM snapshots and file system snapshots. If it's possible to take snapshots with LVM, why does someone take their time to implement it in a file system?

Edit: Is any of them preferred in some situations? Why?

I have made an LVM snapshop by

lvcreate --name snap --size 10G -s /dev/vg00/vm

What command should I write if I want to drop the snapshot, and not keep the changes that have happened since the snapshot?

And what command should I write to roll the changes from the snapshot into /dev/vg00/vm ?

I'm about to perform an experimental upgrade on my CentOS 5 server. If the upgrade fails, I want to be able to back out the changes to the filesystem. This scenario seems similar to the example in Section 3.8 of the LVM HOWTO for LVM2 read-write snapshots - but the example is rather lacking in the actual how-to.

1. How would I commit the changes, merging them back into the original partition?

2. How would I revert the changes, restoring the filesystem back to its original state? Should I assume that I'll need to restart several services, if not outright reboot?

3. Is it possible to snapshot only certain directories on a partition, or is it a partition-wide operation?

I'm trying to figure out how LVM snapshots work so I can implement it on my fileserver but I'm having difficulty finding anything on google that explains how it works, instead of how to use it for a base backup system.

From what I've read I think it works something like this:

• You have an LVM with a primary partition and lots and lots of unallocated freespace not in the partition
• Then you take a snapshot and mount it on a new Logical Volume. Snapshots are supposed to have changes so this first snapshot would be a whole copy, correct?
• Then, the next day you take another snapshot (this one's partition size doesn't have to be so big) and mount it.
• Somehow the LVM keeps track of the snapshots, and doesn't store unchanged bits on the primary volume.
• Then you decide that you have enough snapshots and get rid of the first one. I have no idea how this works or how that would affect the next snapshot.

Can someone correct me where I'm wrong. At best, I'm guessing, I can't find anything on google.

vgdiplay

obu1:/home/jail/home/qps/backup/D# vgdisplay
  --- Volume group ---
  VG Name               fileserverLVM
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  3
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               2
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               931.51 GB
  PE Size               4.00 MB
  Total PE              238467
  Alloc PE / Size       238336 / 931.00 GB
  Free  PE / Size       131 / 524.00 MB
  VG UUID               qSGaG1-SQYO-D2bm-ohDf-d4eG-oGCY-4jOegU