SnapShot behavior with or without SnapReserve enabled

[netappmagic]

NetApp SnapShot uses copy-on-write technology. However, there would be difference in the case of if SnapReserve is enabled or not. 

If SR is not enabled, then there will be no data copies at all (so, not copy-on-write strictly speaking), because new data will always be written to new and empty blocks. 

If SR is enabled, then copying will happen, and blocks will be written to the reserved area before writes, which will also have a performance hit, because the change will be waiting on writes. 

Am I understanding this correctly?

Thanks!

[aborzenkov]

No, you do not. There is no difference in behavior with or without snap reserve.

[AndreUnterberg]

NetApp does not use Copy on write when doing Snapshots. They just point to the old block and declare the condition after snapshot as new Original. So for all new writes after snapshot go to the new Original. There is no copy action.

SnapReserve is an amount of capacity to give on top of your volume capacity.

If you build a volume of 4TB and give a space reserve of 10% the volume size will be 4,4 TB. But your Client/Server will only see a 4TB volume. 

It's just for safety reasons. So your Snapshots cannot fill up your data space.

Kind regards 

Andre

[aborzenkov]

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@AndreUnterberg wrote:

It's just for safety reasons. So your Snapshots cannot fill up your data space.

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Is it something new in C-Mode? In 7-Mode snap reserve did not restrict snapshot “size” in any way, and it was always possible to fill up all available space.

[OGill]

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@aborzenkov wrote:

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@AndreUnterberg wrote:

It's just for safety reasons. So your Snapshots cannot fill up your data space.

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Is it something new in C-Mode? In 7-Mode snap reserve did not restrict snapshot “size” in any way, and it was always possible to fill up all available space.

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As far as I am aware of there still is Snapshot Spill in cDOT. So if you do not configure Snapshot Autodelete the snapshots will spills over into the active file system if your Snapshot Reserve is full.

[AndreUnterberg]

Sorry wrong wording. The snapshots still can spill over into data space. @OGill is absolutely right.

[Anli-A]

Sorry ANDRESCHMITZ but you are wrong.

The snap reserve is puicked up from the volume space.

Hence in your exemple, the active system space will be 4TB - 0.4TB=3.6TB

And the snapshot can still spill the active system space if the space reserve is full.

ref to control the volume space usage:

https://library.netapp.com/ecmdocs/ECMP1368859/html/GUID-77437C70-6983-4BF7-AA01-2E5F7C709059.html 

[imhotep]

„So your Snapshots cannot fill up your data space.„

Not really a SR cannot guarantee that a volume will not be filled up, data will grow into active Filesystem Volumespace and fill it up. SR is just like an Airback for FSs that have potentially high change ratio.

[GidonMarcus]

Hello

SR should not impact perf in any way. all it does is to reserve some space on the AGGR and VOL space accounting. it's does not pre-allocating blocks or anything similar.

Gidi.