Cannot create snapshot - out of space

[bmoreitdan]

FreeNAS 11.2-RELEASE
Issue: After creating a new iSCSI zvol on a newly built system, I created periodic snapshots every 15 minutes. This worked fine for the first 12 hours, then I started getting out of space alerts. There is plenty of available space on the zVol. Note: the zVol is set to 80% of the pool. There is still plenty of space left within the zvol, nearly 50%.

Any idea why I can't create snapshots now? I don't even know where to look. I deleted all existing snapshots and still cannot manually create one. I read somewhere that the problem might have to do with zfs reserving 50% for snapshots, and to check if refreservations is set to none (mine is set to none). I experimented with setting a quota, no luck. I experimented with setting a reservation. Both of these resulted in the same out of space response.

 

[Ericloewe]

There is still plenty of space left within the zvol, nearly 50%.

How is that in any way relevant? It's an abstract bag full of numbered blocks as far as FreeNAS is concerned. How "full" it is is for the application to decide.

How much space is actually needed to store the zvol is the sum of:

• Space needed to store all blocks that have at one point been touched by the application
• Space needed to store all the snapshots, so the latest changes, including TRIMmed deletions, that occur after snapshots
• MINUS TRIMmed blocks that are not part of any snapshots

In practice, you'll find that there is a lot of growth and little freeing.