Snapshot out of space

[Jaques]

Hi guys,

I am new to freenas need some help please. I have one zvol used for iscsi. I allocated 90% of the volume to iscsi lun. I am now getting failed snapshot, out of space. Why is this?

Thanks
Jaques.

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[m0nkey_]

http://doc.freenas.org/11/sharing.html#extents

For performance reasons and to avoid excessive fragmentation, it is recommended to keep the used space of the pool below 50% when using iSCSI. As required, you can increase the capacity of an existing extent using the instructions in Growing LUNs.

 

[Stux]

. I am now getting failed snapshot, out of space. Why is this

Probably because you are out of space?

If you allocated 90% of your volume to a LUN. And then snanpshotted it. And then filled it to 11% and then snapshotted it, you'd be out of space.

Because you only have 10% free BEFORE you started making changes.

And you should never fill a ZFS pool past 90% anyway (it falls off a performance cliff as it switches block finding algorithms at 90% full)

 

[Jaques]

Thanks for the feedback. Freenas is quite a Learning curve. will reduce the size and see what happens

 

[SweetAndLow]

How about don't use a zvol? If this is tripping you up you probably don't need iscsi. It's an advanced feature.

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[Jaques]

Hi All

ok, I think I figured out how this snapshot space thing works. Please correct me if I am wrong. If I have say 3x 3tb drives in Raidz, this give's me 5.2tib usable storage.
If I create a Iscsi Lun of say 500gig and fill this up for argument sake, the total space will then be 1tib after the snapshot ran, correct? or am i still missing it?

 

[Stux]

not quite.

A snapshot initially takes no space. As the data changes, anything you delete/change which was present in the snapshot will still be present in the snapshot in its orginal state, and that is why the snapshot grows.

So, you write 200GB, take a snapshot... no extra space... now you delete the 200GB, the 200GB is still present in the snapshot... so you don't get the space back while the snapshot exists.

Alternatively, you write 200GB, take a snapshot, then modify 50GB of the original 200GB. Now the snapshot is referring to the original 200GB and is thus using an additional 50GB.

 

[Jaques]

awesome thanks for the reply. now it makes perfect sense.