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Convert Thin LUN to Dedupe

rashminder
Occasional Visitor

Convert Thin LUN to Dedupe

Hi Experts,

I'm converting Thin LUN to a Thinly dedupe LUN using tunevv command. Will this cause data to be lost?

Command: tunevv usr_cpg CPG_DD -tdvv SAN_TN

 

...Please suggest...

11 REPLIES
Sheldon Smith
Honored Contributor

Re: Convert Thin LUN to Dedupe

No.

What would make you think otherwise?


Note: While I work for Hewlett Packard Enterprise, all of my comments (whether noted or not), are my own and are not any official representation of the company.
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rashminder
Occasional Visitor

Re: Convert Thin LUN to Dedupe

I have converted a test LUN which was 17% full as per IMC, but after running this command, its 0%

The LUN was not in use, so, I don't know about impact (if any).

I'll be freeing up couple of LUNs tomorrow & test before running on production data.

Dennis Handly
Acclaimed Contributor

Re: Convert TPVV to Dedup

>I have converted a test VV which was 17% full as per IMC

 

What does the CLI say about usage?  Before and after?

Gary_L
HPE Pro

Re: Convert Thin LUN to Dedupe

Keep in mind that the view on a VV space when it's a TDVV vs a TPVV are different.  Becasue we have data in the DDS and the client volume or DDC we show more of a load factored view of the space so you have to be careful in identifying capacity on a per volume basis.

For instance, if you had 3 volumes that were identical deduplicatable data and they were each 10G of space on a 100G volume.  That 10G would be in the DDS and the space for each volume would show aproximately 3.33G each because they are all contributing to the DDS and the load is distributed for the data shared.  If you removed one of thoes volumes, the space in the DDS doesn't change, (still 10G) but the two volumes would now report 5G each.

You will not lose data on any conversion, that should not be a worry.

Gary

I am an HPE Employee
sunshuai
Occasional Advisor

Re: Convert Thin LUN to Dedupe

Can you explain DDC and DDS in detail? DDS is shared with the same VV in CPG, is DDC present in every VV?

Gary_L
HPE Pro

Re: Convert Thin LUN to Dedupe

Correct, the DDS is the shared space for all of the TDVVs in the same COG. DDS can grow up to 64TB.

DDC is just the dedup client volume or the actual VV you created. The DDC holds data as well.

Gary
I am an HPE Employee
sunshuai
Occasional Advisor

Re: Convert Thin LUN to Dedupe

Ok, thanks, I understand that. I have another question.

After enable deduplication and compression.Why all writes with new hashes are written to the private space(DDC) first,not shared space(DDS),when data is seen for second time ,the data is moved from private space(DDC) to shared space(DDS).This will not consume resource?Why cant write DDS in the beginning?Why keep such a big DDC and so small DDS?

sunshuai
Occasional Advisor

Re: Convert Thin LUN to Dedupe

In the past, only deduplication, DDC space is particularly small, only a very small hash of conflict data is stored, why later joined the compression function to make such a big change, a very large DDC and small DDS .

sunshuai
Occasional Advisor

Re: Convert Thin LUN to Dedupe

I have seen a document before saying that there will be an Express Scan before doing the compression. What does this Express Scan use to determine if this part of the data is compressible?

Gary_L
HPE Pro

Re: Convert Thin LUN to Dedupe

sunshuai,

Express Scan is used to prevent wasted CPU cycles, if a page is not compressible then it is written to disk uncompressed.  This is instead of compressing the entire page only to find out the data was not compressible and using resources to gain limited/no return.

I am an HPE Employee
sunshuai
Occasional Advisor

Re: Convert Thin LUN to Dedupe

Thank you Gary, you answered my confusion well.