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Tier % of FAST policy
We would mention the Tier % while creating FAST policy right? like 80% of SSD, 60 of FC etc...
What does it mean by that %? eg. 80% of SSD?
Is that means like 80% of the total capacity of each associated Storage Group (irrespective number of TDEVs in the SG) of can occupy SSD Tier or 80% of each TDEV in the associated Storage Group can occupy SSD Tier?
M_Salem
213 Posts
1
June 11th, 2014 00:00
Hi Vibin,
The first statement is correct, however, it needs more clarification..
Actually movements (promotion or demotion) will be on extent group level (7.5 MB) irrespective of the TDEV size so in your example. if we have TDEVs of total size 250 GB presented to your server and associated with that policy that states 80% of this group can be promoted to SSD, FAST-VP allocation compliance algorithm will ensure that no more than 80% of total SG size can be promoted to SSD BUT that does not mean that 80% will be promoted. The other FAST-VP intelligent-tiering algorithm will actually control this.
Based on performance collection for your extent groups (7.5 MB) a performance score is calculated and compared to promotion/demotion threshold for each tier, The FAST-VP intelligent-tiering algorithm will make a decision to promote or demote your extent groups based on this comparison. If it is higher than the promotion threshold then the extent group will be promoted to higher tier, if it is lower than demotion threshold then it will be demoted to the lower tier. If it is in between both threshold then it will not be demoted or promoted.
For more info about FAST VP theory of operation, Please read this awesome whitepaper:
https://support.emc.com/docu31003_FAST-VP-for-Symmetrix-VMAX-Theory-and-Best-Practices-for-Planning-and-Performance.pdf?language=en_US
Hope that helps
Mohammed Salem
Fenglin1
2.1K Posts
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June 10th, 2014 02:00
In your case, 80 percent of the configured capacity of the associated storage group can be placed in the EFD(SSD) tier, 60 percent on the FC tier, If the thin devices in the storage group are fully allocated, then 80 percent of the storage group’s capacity will always be located on the EFD tier, and so on for the remaining FC tiers.
M_Salem
213 Posts
1
June 10th, 2014 02:00
Hello Vibin,
Welcome to EMC Community.
The way policy max_sg_percent works is as follows: (taking your example into consideration)
1- (max_sg_percent of 80% for SSD tier) applies to the total TDEVs size of all SGs assicated with this policy.
2- It will not necessarily occupy 80% of each TDEVs in SSD tier. The policy considers FAST VP available capacity per the whole tier so let us assume that we have SSD tier which has 2 SSD pools.
Total space available in the pools (ex. each 750 GB) =1500 GB,
PRC set of each pool=10%
FAST available capacity =1500 - 0.1*750-0.1*750= 1350 GB.
so back to yoru example. the associated SGs can occupy up to 80% of the FAST available capacity (in my example 0.8*1350GB)
to get FAST VP available capacity, you can run this command:
symfast -sid xxx list -demand -vp -technology efd
Hope that helps
Mohammed Salem
Vibin1
62 Posts
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June 10th, 2014 03:00
Thanks Salem and Fenglin.
You mean to say that,,,,,percentage capacity is the X% of total available capacity in the Tier for FAST and also that rule always be applicable to all TDEVs in the associated Storage Group. Right?
and not as X% (here 80%) of total SG capacity (what ever may be size) and X% (here 80%) capacity of each and every TDEVs in the same SG.
Vibin1
62 Posts
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June 10th, 2014 08:00
Consider I have a policy Gold where I defined 80% SSD, 50% FC and 30%SATA. And I have associated that policy with a SG TESTSG. I have 2*100GB volumes and 1*50GB volume over there.
Here my total SG capacity is 250GB (2*100 + 50GB). Here is my question, what does it mean by 80% here, Is that means 80% of my SG size (200GB) can occupy SSD Tier irrespective each TDEV size? ie. It could be a single TDEV or multiple TDEV sizes but maximum is 80% of that SG
Or 80% of each and every TDEV can occupy SSD Tier?
Or the SG TESTSG can occupy the 80% of available SSD Tier capacity?
Which statement is right among above 3?
Vibin1
62 Posts
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June 11th, 2014 02:00
Thanks a lot.