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 Post subject: Question regarding AO and Free Space on Lowest (NL Tier)
PostPosted: Sun Feb 04, 2018 11:41 pm 

Joined: Sun Feb 04, 2018 6:39 pm
Posts: 3
Question regarding AO and Free Space on Lowest (NL Tier)

Apologies if this is an obvious question but have scanned the Documentation and cant see a direct answer...though doesnt mean its not there - may have just missed it :-(

I have a few AO Policies configured

Policy 0

SSD
FC
NL

Policy 1

FC
NL


I have noticed that the NL Tier appears to be consuming a lot of space and is creeping to the 90% consumed mark. There are no growth warnings configured for any of the CPG or AO configurations. The entire Array is provisioned using AO configurations.

What I would like to know is even though there are no growth warnings configured, can I expect AO to manage this space accordingly and if there is a shortage of NL Disk to keep it on the higher FC tier which has plenty of space? I am not sure if I need the specific warnings (wouldn't use limits) set on the CPG or the AO Configuration to make this happen? Kind of hoping that AO would be clever enough not to move data to tiers where these is little or no space. What amount of space would it consume on the NL tier before it stopped trying to move?

Any advice would be appreciated. Thanks in advance.


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 Post subject: Re: Question regarding AO and Free Space on Lowest (NL Tier)
PostPosted: Mon Feb 05, 2018 1:38 am 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 590
AO will move data as long as there is free space. The recommened setup for AO is that all volumes have FC CPG as user (and snap CPG) so that all new data will to FC and it isn't a problem that both SSD and NL is ~100% full. In any AO setup, you need to ensure that you have enough free space on all tiers where new data is written. How would the 3PAR know what would be "enough" space on each tier?


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 Post subject: Re: Question regarding AO and Free Space on Lowest (NL Tier)
PostPosted: Mon Feb 05, 2018 6:04 am 

Joined: Sun Feb 04, 2018 6:39 pm
Posts: 3
Thanks MammaGut for replying -appreciated.

As per the first line in the 'Space available in the tiers' section of the 'HPE 3PAR StoreServ Technical White Paper' it states that AO's first job on run is to 'consider available space on tiers'.

Space available in the tiers
Quote:
When AO runs, it will first consider available space in the tiers. If a tier has a CPG warning limit or a tier maximum value configured and the tier
space exceeds the lower of these limits, AO will move regions to reduce space consumption below the limit. (Note: If the warning limit for any
CPG is exceeded, the array will generate a warning alert). If space is available in a faster tier, it chooses the busiest regions to move to that tier.
Similarly, if space is available in a slower tier, it chooses the idlest regions to move to that tier. The average tier service times and average tier
access rates are ignored when data is being moved because the size limits of a tier take precedent.

And similar in the 'Adaptive Optimization for HPE 3PAR StorServ Storage' (page 19)

Quote:
StartAO has now been initialized, and the needed data collected or calculated. The next phase will schedule region moves. The first region moves
considered by AO are based on available space. AO will check the CPG’s in the AO configuration for free space. If one or two of the CPGs do not
have sufficient free space, AO will attempt to move regions between tiers to create free space. If sufficient free space is unavailable on all tiers, AO
will stop. This is the phase where AO will consider any tier minimum or maximum values as described earlier in the section titled “Minimum and
Maximum Space within a Tier”. AO will schedule region moves, if necessary, to satisfy any minimum or maximum settings.


As the Best Practice also states that AO Configurations should be run together as they will calculate what needs to be moved at the same time but moves will occur serially - it would kind of sound like a space check would occur before moving each so there would be no simultaneous operation that could fill the tier.

However, all that said, my question was relating to if there are NO Warning limits set will it just go to 100% (or as close to it as it can) usage of the NL Tier until it would essentially resort to swapping around chunks as opposed to move more down?

What I am essentially trying to get to is if I dont add any further NL Disk and it goes to 100% can I just let AO manage this without issue - albeit of course accepting that it wouldnt be entirely balanced and I would have data sat on FC (of which I have a lot free) that probably shouldnt? Like I mentioned before this array has purely AO CPG's and nothing dedicated to the NL tier.

As you mentioned all my User CPG's for the Policies are sat on FC tier, however I noticed 2 rogue VV's that someone has placed on the NL. Is it fine just to tune those 2 on to the FC CPG of that AO policy?

Your advice and time is much appreciated.


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 Post subject: Re: Question regarding AO and Free Space on Lowest (NL Tier)
PostPosted: Mon Feb 05, 2018 2:48 pm 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 590
gibwanna wrote:
However, all that said, my question was relating to if there are NO Warning limits set will it just go to 100% (or as close to it as it can) usage of the NL Tier until it would essentially resort to swapping around chunks as opposed to move more down?

With no warning (or t2max) AO will not stop until NL tier is 100% if there is enough inactive data on the system.
gibwanna wrote:
What I am essentially trying to get to is if I dont add any further NL Disk and it goes to 100% can I just let AO manage this without issue - albeit of course accepting that it wouldnt be entirely balanced and I would have data sat on FC (of which I have a lot free) that probably shouldnt? Like I mentioned before this array has purely AO CPG's and nothing dedicated to the NL tier.

As long as no other volumes are writing new data to the NL tier it shouldn't be an issue. If AO fills NL tier to 100% and you have "rogue" NL volumes, they will not be able to grow.

The same was/is true for SSD. We're not talking many years ago when even the big systems might only have 8x 100GB SSDs per node pair and those were almost always 100% full. Only difference there was that if you had rogue volumes, they could use FC or NL if 100%. NL doesn't have anything else.


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 Post subject: Re: Question regarding AO and Free Space on Lowest (NL Tier)
PostPosted: Tue Feb 06, 2018 5:43 am 

Joined: Sun Feb 04, 2018 6:39 pm
Posts: 3
MammaGutt - thanks for the clarification. Great Info


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