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Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

Just started working on the NetApp products and doing a lot of reading about them.  Something I am trying to wrap my head around is all this information dealing with thin provisioning, fractional reserve, and snapshots, that if not configured correctly can make LUNs go offline.

I need help understanding if what I did may have created a problem.

I have multiple volumes with iSCSI LUNs connected through SnapDrive to Microsoft VM guests.  For the Exchange servers and SQL servers, SnapManager is being used to take hourly snapshots and then SnapMirror replicates to another site.

The volumes are larger then the LUNs with available space ranging from 8GB to 70GB throughout the different volumes.

I decided to change the volumes from space guarantee to space setting NONE with 100% in the fraction reservation field (through the System Manager).  The LUNs are all space reserved.

Can someone help me to understand if this is a good configuration or if I am looking at a problem down the line?  Is there better ways to configure things while thin provisioning, because I've read that one of the benefits of using a SAN storage system with virtualization is the benefits of maximizing the storage the best way.

This helped to increase my aggregate sizes to 500GB, 800GB, 1.36TB, 890GB, and 3.2TB.

Re: Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

Hi SANMAN, welcome to the community.

There is a discussion in regards to fractional reserve. I have included the link http://communities.netapp.com/groups/chris-kranz-hardware-pro/blog/2009/03/05/fractional-reservation--lun-overwrite

It is really good.

In regards to your question, about changing volume reservation and lun reservations after the fact. I'm not sure if it will cause any problems. As in all IT scenarios, it all depends. If you have enough space in your aggregate and the volume is configure correctly, then your LUN and hopefully your MSSQL/Exchange shouldn't be in any jeopardy. I think for the NetApp SNAPdrive, there is a setting when you attached your drive that you reserved certain space for snapshots and that effectively decreases your available space. For SNAPdrive 5 or 6, I think that you can configure some sort of thin provisioning from the SNAPdrive itself.

I tend to be more on the conservative side, hence my configuration of volume - space guarantee, LUN - no space guarantees. i haven't changed the configuration after everything was set. I know that you can even configure it as such where there is absolutely no space guarantees on both volume and lun level. I think it is just a matter of how comfortable you are with thin provisioning and knowing your data growth.

Hope I was able to help.

Re: Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

http://communities.netapp.com/message/5765#5765

another good discussion on space reservations and fractional reserved

Re: Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

On NetApp you take the basic unit, the RAID group, and combine them into aggregates.  In a rough sense, an aggregate is a pool.  Within an aggregate, you can create a logical construct called a volume.  Within the volume, you can create qtrees, luns, and files.

At the volume level, you can set a few different reserves.  If you choose not to, your volume is considered “thinly provisioned”.  Likewise, on the LUN you can choose to reserve space in the volume for the declared size or not.  If you choose not to, it’s “thinly provisioned”.

Thin provisioning is cool.  I have a 1GB volume on a simulator, and within that volume I have two 1GB Luns.  The sum of the data on the LUNs is 2.3GB, but I’ve deduped that so it only really consumes 1.5GB.  The volume still has .5GB of space available.

Thin provisioning is a double edged sword however.  I have to carefully monitor my space consumption as I am overcommitted 100%.  I also have to watch my dedupe ratios; if they were to go south, I’d have a problem.  That right there is the thrust of thin provisioning; yes you can save a lot of space, but you better be monitoring it and prepared to take action.

Oh, wait a minute; what about those Qtrees?   Within a volume, a Qtree is a logical construct to which you can apply a quota.  If all my luns are all in qtrees, then ...  Think about it…

J

Re: Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

Hi,

One more interesting thing re thinly provisioned volumes & fractional reserve:

Not sure if it has been fixed in ONTAP 7.3.3, but definitely until 7.3.2 it behaves as follows:

When volume guarantee is set to none, fractional reserve is automatically set to 100% & cannot be changed. The volume, however, behaves like fractional reserve is set to 0% (i.e. fractional reserve does not kick in).

Regards,

Radek

Re: Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

Unfortunately, not quite. It still tries to fulfill reservations, which means that if autogrow is enabled it will try to grow (virtual size of) volume.

Where it kicks in, when you create FlexClone; it inherits autogrow option and fun starts. It has been discussed recently (lost thread reference), where it apparently could fail snapdrive clone operation.

As far as I can tell today there are two safe settings:

volume guarantee volume + LUN space reservation enabled

volume guarantee none + LUN space reservation disabled

Anything in between is bound to have unforeseen side effects.

Re: Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

Hi SANMAN,

Space reservation is never used unless the available space in the volume has been completely used. You can set this value lower if you know your environment and choose to do so.  Space reservations don't really impact performance, they just give you a cushion against this condition.

I personally think that fractional space reservations less than 100% is always comes handy in order to grab more available space in the volume.

If the LUN space guarantee is turned off, the storage system still tells the host that the LUN is there but never actually carves out the space until you starts to sill the data into it, i.e. no space is consumed on the system until data actually starts coming into the LUN. At this point, the LUN is thin provisioned.

Pls go through the NetApp TR on this:

http://media.netapp.com/documents/tr-3483.pdf

Just be sure that you don't confuse volume gurantees with lun space reservations. Thin provisioning is typically done at the volume level and is done with volume gurantees. So, for example, if you set gurantee to 'none' you will only chew up space in the underlying aggregate as you consume it in the volume as opposed to the default where the space in the aggregate is allocated at volume creation time.

Hope this helps.....

Cheers....

Mohit....

Re: Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

Hi,

We definitely have full agreement on this one:

As far as I can tell today there are two safe settings:

volume guarantee volume + LUN space reservation enabled

volume guarantee none + LUN  space reservation disabled

I don't actually see much sense in implementing any other combination of thick / thin provisioning. The first one leaves free space management at the LUN level, whilst the second one genuinely moves it to an aggregate level (providing 'nominal' sizes of all thin LUNs are big enough to accommodate 'any' data growth)

It still tries to fulfill reservations, which means that if autogrow is enabled it will try to grow (virtual size of) volume.

Where it kicks in, when you create FlexClone; it inherits autogrow option and fun starts. It has been discussed recently (lost thread reference), where it apparently could fail snapdrive clone operation.

I haven't noticed the issue, maybe because I just don't use thin volumes with thick LUNs. But I do remember seeing the thread you are refering to.

Cheers,

Radek

Re: Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

Thank you all for explaining this as best as possible.  I think it's going to take time to learn this technology and wrap my head around it.  This seems to be an ongoing topic of conversation.

The other discussion Jeddgo123 posted up helped me more.

The ongoing thing that is still confusing is the fractional reserve part.  When setting this, where is the actuall space set aside at?  In the volume or at the aggregate.  So if I create a 50GB volume with volume guarantee and 100% fractional reserve and snapshot reserve 10% that gives me 45GB available in the volume and 5GB reserved for snapshots.  Now I create a 25GB LUN that is also guraranteed, which will then give me 20GB free in the volume.  Since fractional reserve is 100% will I get an error that it cannot create the LUN, because there isnt't 50GB available (25GB for LUN and 25GB fractional reserve) in the volume or will I see the extra space reserved at the aggregate level.

Please bare with me as I talk this out.  Thanks.

Re: Trying to Understand Thin Provisioning Volumes With iSCSI LUNs and Snapshots

Another topic that frequently comes up in the SAN, iSAN conversation is how space reclamation works in conjunction with thin provisioning. We provide this functionality for SnapDrive for Windows and the following link may be useful for anyone using Symantec Veritas Foundation Suite.

http://www.netapp.com/us/library/white-papers/wp-7111.html