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iwStack: How much space do snapshots take up?
cirrus_cloud
Member
in Help
If mirroring a volume / image in iwStack, does a snapshot occupy the same amount of fixed space as the image? So if the root volume or some other volume is a fixed 20 GB but only 5% of it is being used, how much space does the snapshot occupy? Is the snapshot using 1 GB or 20 GB in that case?
I tried reading here: https://cwiki.apache.org/confluence/display/CLOUDSTACK/VM+Snapshots. But I'm still not sure after reading that.
Comments
Did you send in a ticket regarding this question to the appropriate parties?
Been a while, but I think the snapshot only consists of the data actually used, and it's compressed.
Yes, I already sent in two today, but I didn't want to spam them. Also, I found the other topics on iwStack here on LET to be helpful. I thought this could act as a reference for others.
My question was
The response was
I am not entirely sure what that means, and I don't see anything explicit on the issue on the wiki for apache cloudstack.
So if Nekki is right, then if I have an image where N=100GB (partitioned space) and M=7GB (used space), then each and every snapshot will take up just M space. If I want to have 4 backups / snapshots of an image, then it's 4 * M.
An image will use the entire volume, not the data inside that volume. So if it's a 10GB volume with only 1GB used then the image will still be 10GB.
That is how I understand it anyway.
Unless they do some sort of sparsing of the data then yes, it'll be whatever the max usage you've ever had just because those blocks are 'dirty'.
Francisco
As far as i can remember (It's months since i did that and i'm old
, if you export and download a volume, it's compressed but only the real used data is exported. Never had snapshots... so i can only speak for exported instance volumes.
Yeah, I'm not sure if it's the same if they save the snapshots.
Raise a ticket mate, only way you'll get an answer....unless.....
I invoke @maounique
or simply try ;-)
as other said it most probably is the full size, as they wont be able to determine from outside what parts of the image are actually used and which are just dirty blocks.
even if they start with a sparsed image which could result in a considerably smaller snapshot, that image will grow over time by your system writing and deleting files.
most filesystems don't use old dirty blocks after a file deletion but unused ones for new writes if available. so you will see the image growing despite not needing more space inside at all.
if they do use some kind of compression on the snapshot it might help if you do overwrite the unused dikspace inside your container with zeros ( dd if=/dev/zero of=/tmpfile ... ) until disk is full, then remove tmpfile. this is just to get rid of the dirty blocks and have the snapshot feature hopefully compress the zeroed part.
if sparsing works at least, you might consider having a look into using xfs, as this does overwrite/re-use dirty blocks first instead.
No compression takes place, even at downloading, the images are 1:1.
The problem with compression is that it takes CPU and extra I/O and most people take snapshots at the top of the hour, it would be a disaster if they all start using cpu and I/O in the same time, especially on SAN connected zones, even if some bw on the secondary storage will be saved. We upgraded the links to secondary storage so that is not an issue now.
And I've just realized that you're asking about iwStack. For some reason my brain read LunaNode, so that's what I was testing with...hopefully the same is true with iwStack
Thanks for the replies. Support replied and @Maounique have confirmed that images are the same size. This is Apache cloudstack as mentioned in the original post.