STH Forum ZFS Snapshots - guess how much data

Patrick

Administrator
Staff member
Dec 21, 2010
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I just wanted to take a quick poll.

STH forums run in their own Ubuntu VM using ZoL with Proxmox/ Debian as the base hypervisor distro. The volume has a total size of 40GB.

Here is my crazy question for you: day to day snapshots how much data changes?

Code:
zfs send -nv -i sasarray1/vm-102-disk-1@sthforum20151209 sasarray1/vm-102-disk-1@sthforum20151210
What is the total estimated size ______?
I have to say, I was a bit surprised at first. It does at least give some idea how much data is changing on the disks daily.
 

Jeggs101

Well-Known Member
Dec 29, 2010
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A few logs and a few text database updates. How big could that be? 10MB? 50?
 

Patrick

Administrator
Staff member
Dec 21, 2010
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Ending the suspense - it turns out the forums generate 600MB or so per day of snapshot delta or about 1.5% of the total volume size (but there is a lot of free space.)
 
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Mike Bailey

Member
Sep 24, 2015
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@Patrick: What stack are you running for the actual forum software? If I had to guess, it's unlikely the VM itself generates much in the way of anything beyond incremental DB changes and some logs.

Just guessing, logs might be no more than 100 MB a day, if that. Database changes? Easily not a lot. If you look at the actual underlying data files, most DB engines do a very good job of making sure they can make changes in place without having to split the pages in an index.

That being said, 100 - 200 MB of writes to a database, especially if it's new content added on, will only amount to a small amplification factor in terms of disk churn.