topic Re: Doubt about SNAP RESERVE and SNAP MIRROR in ONTAP Discussions

Doubt about SNAP RESERVE and SNAP MIRROR

NAYABRASOOL — Thu, 05 Jun 2025 06:07:14 GMT

Hi Experts,

I have a doubt regarding SnapShot reserve how it works in case of THICK PROVISIONING and THIN PROVISIONING please help me understand this in detail and Snap mirror as we know we have Volume based and Qtree based snap mirror and is there anything called FILE BASED and BLOCK BASED Snap mirror ???

Please help me to understand

Thanks,

Nayab

Re: Doubt about SNAP RESERVE and SNAP MIRROR

naveenkumar_e — Tue, 19 Mar 2013 08:38:45 GMT

Hi,

Provisioning is at volume level. Thick Provisioning makes you reserve the entire volume size (you specify while creating) from the aggregate. Where as Thin Provisioning doesn't reserve any space from the aggregate. But as it fills up, it starts occupying the space from the aggregate. The space limit in this case would be the size you mentioned while creating the volume.

You can choose the type of provisioning by changing 'space reservation' parameter in volume options.

space reservation: 'volume' for Thick Provisioning; 'none' for Thin Provisioning

SnapReserve holds same in both the cases as it is part of the volume. Whatever the percentage you provide, it will be applicable.

Eg: Consider vol_thick 100 GB total size and snapreserve 20%, it reserves 100 GB from aggregate of which 80 GB will be for volume and 20 GB will be for snapreserve.

And for vol_thin the same policy applies, you cannot write data into volume after occupying 80 GB space from aggregate.

Anyway, the size limit in case of thin provisioning varies with parameters like 'autogrow'.

QSM is inode/file based mirroring and VSM is block based mirroring. There are many differences between VSM and QSM.

Thanks,

Naveen

Re: Doubt about SNAP RESERVE and SNAP MIRROR

BALAJI_VENKATRAMAN — Tue, 19 Mar 2013 09:17:22 GMT

That is correct...thin provisioning is done at aggregate level

The volumes in a thin aggregate still have the same 20 % limit (or whatever you set in your infrastructure) for eg when you create a 100 GB thin volume,the reserve would be 20 GB only beyong which your alerts would start popping up for snap overflow.

The only difference in THIN provisioning is that you have to additionally monitor the utilization of the aggregate and add space as and when needed.

Out of all the differences b/w VSM and QSM,VSM is usually faster than QSM as it carries the entire blocks in one shot.QSM is slower (though roughly can be compared to Snapvault) as it has to migrate data at the qtree level making it slightly slower.

Again in simple words to answer your question - VSM is block based mirroring and QSM is file based mirroring

Message was edited by: Balaji Venkatraman

Doubt about SNAP RESERVE and SNAP MIRROR

NAYABRASOOL — Tue, 19 Mar 2013 09:58:08 GMT

Ok what if my Snapmirror is a Volume based one and it has Qtree as well ?? What it is called as ?? a FILE SM as it has qtree's OR BLOCK SM as itself is a volume ??

Re: Doubt about SNAP RESERVE and SNAP MIRROR

NAYABRASOOL — Tue, 19 Mar 2013 10:02:28 GMT

And how about Snap reserve in case if it is getting filled up say it is occupying the whole 20% then will it occupy the space from volume say i have not enabled AUTO GROW or AUTO DELETE what happens in this case of THICK n THIN volumes types ???

Re: Doubt about SNAP RESERVE and SNAP MIRROR

naveenkumar_e — Tue, 19 Mar 2013 10:17:26 GMT

Though a volume has qtree inside it, if you are running Volume based snapmirroring, it is BLOCK level transfer.
For Volume snapmirror, mirroring will be initialized by specifying 'snapmirror initialize -S sourcefilername:/vol/volume destfilername:/vol/volume

For Qtree snapmirror,mirroring will be initialized by specifying 'snapmirror initialize -S sourcefilername:/vol/volume/qtree destfilername:/vol/volume/qtree

So you can easily identify in the snapmirror status..

Re: Doubt about SNAP RESERVE and SNAP MIRROR

naveenkumar_e — Tue, 19 Mar 2013 11:08:58 GMT

If snapreserve gets full, it starts occupying space from the volume,so you need to monitor the space usage of snapreserve closely. If the volume gets full, it may go offline.

If you do not enable 'autogrow' or 'autodelete', the volume gets offline when it gets full in both the cases.

These are the options to avoid volume going offline