How does FAST move the extent?

hi evgenym,

Please consider moving this question as-is (no need to recreate) to the proper forum for maximum visibility.  Questions written to the users' own "Discussions" space don't get the same amount of attention and questions can go unanswered for a long time.

You can do so by selecting "Move" under ACTIONS along the upper-right.  Then search for and select: "Symmetrix Support Forum" which would be the most relevant for this question.

Thank you! I moved it. Should I do it for each topic? Have I created in wonrg flow, right?

Ok, this is clear for me I though the same but was not sure, so VMAX can promote or demote the extents if pool has no space. But what about first part of question, I'm totally confused. As I remember from VMAX course if array has strong rule (the sum for all tiers in policy equal 100% in my example 10-10-80) it will keep the written extents according this one. For instance we have 100gb allocated device and written 50 gb, so according policy vmax will write 5 gb on EDF, 5 gb on FC and the rest on SATA 40gb (80% of 50 gb). But why I dont see any written extents on FC pool, I have no idea

For the first part of questions, its really strange for the extent to be promoted directly to EFD. It could be possible that the 10% allocation for FC tier is already full for the policy to which this device is associated or FC pool has already hit 90% of its total capacity.

Another possibility is that the extent was active enough to be promoted to EFD directly.

For instance we have 3 pools - EDF, FC and SATA. and free space on them EDF - 1 gb, FC - 5 gb and SATA - 100gb. and the same FAST policy - 10% EDF - 10% FC and 80% SATA. I bind new volume 100gb to FC pool (Max. Subscription limit allows me to do that and don't think about PRC for this example) and host does write to volume and within 10 min all 100 GBs have been written to volume. Of course almost all of extents will be written to SATA pool (am I right?)

Right, all new writes will go to bound pool unless you have -vp_allocation_by_fp parameter set.

If tiers are full then promotion cant be performed but demotions continue to happen. VMAX code polls sub-lun statistics from all the devices in SG and generates threshhold per tier. Symm calculates promotion score and demotion score for each extent per tier.
Based on this score and threshold, VMAX enginuity knows which extent to demote and promote. If there is not enough space in the tier to move an extent then the threshold for promotion might increase, thus allowing extent with the highest promotion score to be moved to the efd tier.

feel free to add and correct me if i have missed something.

regards,

Saurabh Rohilla

Ok, there is no flow which can be defined to move extents from one pool to another pool and also this is not necessary to move extent should into the FC or bia FC to EFD, this is completely depends on IOPS and according to that extent can move directly from SATA to EFD or FC!! Possibility is that the extent was active enough to be promoted to EFD directly.

Please feel free to add and correct me if i am wrong!!

hi EvgenyM,

if you have other threads created in your personal space, please move them too. For new threads, just go to the related space then click new discussion.

Hello!
Yes, I can agree with you that not needed to keep the tracks on FC pool, but from a course I remember if VMAX see strong rule (sum is 100%) it will place all extents according this policy (for my example 10% should be on EDF, 10% on FC and 80% on SATA) so your answer is not clear for me, I can understand you, but I dont understand the behavior of VMAX in this case, seem you too if you, can you tell me the document which describes that? I couldn't find it on EMC support site.

Hello!

Sorry but it is not possible, if FAST has strong policy it should follow of this policy

FAST operates at the SG level not the device level (unless you have an SG with a single dev in it), devices within the SG may have different allocations but as long as the group percentages fit the policy it will be within tolerance.  

Hi Chris!

Seems I disagree with you, what do you mean when you say FAST operates at the SG level? Actually you just associate SG with proper FAST policy, but FAST mechanism works on sub-lun level with extents. (actually it calculates the performance for each block 64kb each one - this  is minimal entity which VMAX operates (slot, IO rate and so on)) and then migrate the groups of extents (10 items, about 8 Mb). So I would say each volume should follow of this policy, am I right?

or FAST operates only one entity like SG?

Hi again Chris!

I'm not sure but if FAST operates one entity (SG) as you mentioned, does it mean that VMAX looks on the volumes within in one SG like one wide device with a lot of extents and then migrate them according rate of IO for each one?

so for example that to be clear. We have one SG and three devices within A,B,C, and extents for each one:

A -> A',A'',A'''

B -> B',B'',B'''

C -> C',C'',C'''

Does it mean that FAST sees this only like that:

A',A'',A''',B',B'',B''',C',C'',C'''  and in this case it makes sense to work as I described: VMAX can move extents like this:
SATA: A',A'',B',B'''

FC:  nothing

EDF: A''',B', C',C'',C'''

Am I correct?

Thanks!

You both are correct. As Chris said, FAST VP works on SG level, yes it does work on SG level to calculate the policy allocation percentage and extent level to measure performance as you pointed out.

So if a policy is created to have 50% allocated to SATA and its applied to a SG. Then the 50% of the total SG capacity (excluding pinned devices) will be moved to SATA not necessarily 50% of each device in that SG.

regards,

Saurabh Rohilla

Hello!

Please tell me does it mean that if I have strong policy and not enough space for allocation on FC pool for instance, then Enginuity will not follow the rule, am I correct?

As I said in my reply on Oct 16, promotion & demotion are based on the threshold scores for each tier. If the tier is Full, there wont be any promotion (even if the policy is strict).

For e.g. You have 1 TB of total SG capacity and your policy says 100% to EFD (total size of EFD tier is 500 GB) and 1% to FC tier (bound pool). What will happen in this case? Will FAST move 1 TB to EFD that has only 500 GB?

regards,

Saurabh Rohilla