Flash Cache

[mdan_06]

How do i expand the cache capacity of the 3Par using flash cache.

Please let me know how it can be achieved.

Thanks

[sbhat09]

Hello @mdan_06,

Controller cache of 3PAR is fixed. For 8200/8400, it is 64GB per node pair and 8440/8450 it is 192GB per node pair. You can't expand the controller cache.

Flash cache is an additional, lower-level cache that you can create by reserving some portion of flash/SSD drives. Make sure you have sufficient free space in SSD drives. If it is a 4 node or 8 node system, there must be SSD drives (with free space) available in connected cages of all node pairs. Free space available must be more than the amount of flash cache you want to create.

Then you can create the flash cache using the command 'createflashcache <size>'. You can do it from SSMC as well.

Regards,
Srinivas Bhat

[mdan_06]

Thank you.

So, do you recommend Flash cache? 

Pro's and con's if any.. before i enable it.

Thanks in advance.

[sbhat09]

Hello @mdan_06,

I certainly recommend flashcache for 10x faster disk IOps, especially data writes. Though your volumes are on FC/NL CPGs, original data writes will happen on SSDs. Later th the data moves to the disks where they actually belong.
Pros: You will get SSD write performance for all volumes in the system.
Cons: As much flashcache you reserve, that much SSD space will not be available as usable space.

There are no other important impacts or differences with the usage of flashcache.

Regards,
Srinivas Bhat

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[sbhat09]

@mdan_06, additionally, I thought the below information will be handy for you:

The command to create flashcache is create -> createflashcache <size>[g|G|t|T]
If you don't mention the unit [g|G|t|T], system will assume it as MiB.
Example, the command to create 128GiB of flashcache may be:
createflashcache 128g or createflashcache 131072

The size of the flashcache must be a multiple of 16384MiB (or 16GiB).
Minimum size is 64GiB. The maximum size of the flash cache is based on the node types, ranging from 768GiB up to 12288GiB (12TiB).

You can create flashcache from SSMC GUI as well. Option will be under Main menu >> Optimization >> flashcache

Regards,
Srinivas Bhat

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Note: All of my comments are my own and are not any official representation of HPE.

[sbhat09]

Hello @mdan_06,

Was that explanation helpful to you?

Regards,
Srinivas Bhat

[mdan_06]

Hello Srinivas,

Yes. Thank you.

[sto_dts]

Hi sbhat09,

I am trying to re-create the array flash cache after removing it, but it won't let me for a silly reason.

Here is the cache before deleting it:
3parname  cli% showflashcache
-(MB)-
Node Mode State Size Used%
0 SSD normal 1327104 99
1 SSD normal 1327104 100
2 SSD normal 1327104 99
3 SSD normal 1327104 100
-------------------------------
4 total 5308416

And when I tried to re-create it, I keep getting an error:

3parname  cli% createflashcache 5308416
Error: size must not exceed 4194304 MB

I have looked over the internet if there is a way to get this to work, but unfortunately, I cannot create the previous size of the flash cache that we had on this particular 3PAR. It is an 8440 3PAR model.

[sbhat09]

Hello @sto_dts,

Is there expected amount of SSD space available to create flashcache?

Regards,
Srinivas Bhat

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[sbhat09]

Hello @sto_dts,

If you have enough SSD space and still facing the issue, please try to create from the GUI (SSMC).
Main menu >> Optimization >> flashcache

Let me know how it goes. If you still face issues, let me provide an alternate.

Regards,
Srinivas Bhat

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Note: All of my comments are my own and are not any official representation of HPE.

[sto_dts]

Hello Srinivas,

Our vendor succeeded to create the flash cache, but not with the full SSD size that we have, they are now working internally to see what could cause the issue and not be able to create the flash cache as the previous size we had..

Meanwhile, I am writing this reply to you, they were able to create the flashcache as it was before, and I am waiting on their response to see how they have done it.

P.S. You mentioned an alternative way of doing it, could you please share your thoughts as well, to have it for the future if the issue occurs again?

This re-creating of the flash cache was done due to spam alerts of Flash Cache I/O times being slow on nodes

[sbhat09]

Hello @sto_dts,

That is great.! Good to know that it worked. Thanks for updating me here.

Alternately, if space was the problem I thought of advising you to create lower amount flashcache and test it. Or clear some space on the SSDs and then try to create flashcache.

If the command was the issue, I thought of entering the alternate command by defining in alternate units like 'G' or 'g' (in GiBs).

Regards,
Srinivas Bhat

If you feel this was helpful please click the KUDOS! thumb below!
Note: All of my comments are my own and are not any official representation of HPE.