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VNX7500 Read and Write cache values
Hi ,
I have gone through few post regarding the same but still have confusion . Even searched for primus EMC267304 but didn't find it .
We want to change the read and write cache values as we have free SP memory .As per EMC best practice we want to reconfigure it .
We have VNX7500 96GB model i.e 48GB per SP
49152MB (48GB Per SP) - 28438MB reserved for system memory per SP = 20714MB available for user to allocate to read and write cache per SP
Now any suggestions about the values which is at per with EMC that I can set for read and write cache from the available 20714 MB
Thanks in Advance
Cheers
Sutirtha
halls1
25 Posts
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April 14th, 2016 06:00
See page 8 (1st page of BP's) of the VNX Best Practices Guide (for VNX1).
https://www.emc.com/collateral/white-papers/h12682-vnx-best-practices-wp.pdf
ZaphodB
195 Posts
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April 14th, 2016 07:00
The stock EMC position on this has been to allocate a minimum amount to read cache and the lions share to write. Their reasoning being that even maxing out the read cache is unlikely to result in a high percentage of read hits for most workloads, but running out of write cache causes forced flushes which are terrible for write response times, and also bad for read response times.
The minimum amount for the arrays I have ( a bunch of 5300s) is 300MB, and I end up setting the value right there.
HKHK
15 Posts
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April 14th, 2016 19:00
Hi Zaphid ..
Thanks for the reply . My confusion how much should be allocated to read cache and write cache . We have 20714MB left which needs to be allocated to read and write . We have 48GB per SP model .
HKHK
15 Posts
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April 14th, 2016 19:00
Hi Scott,
Thanks for the reply .. The document says max 1024MB for read cache rest to write . Now in our case we have 20714MB and if we give only 1024 MB to read then we have around 19690MB left
But as per EMC for 96GB model(48GB per SP ) the max amount of write cache you can allocate per SP is 16600 MB . SO in this we will have unutilized SP which is not EMC recommended.
halls1
25 Posts
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April 14th, 2016 21:00
I don't recall the details for max write cache. Where do you see 16600 MB as the max?
There is nothing wrong with configuring more than 1024 MB of read cache. It is just generally not necessary, unless the array workloads are very read intensive.
Remember that best practices are not the only practices. They are general guidelines to address the majority. As stated in the BPG, "Guidelines can and will be broken, appropriately, owing to differing circumstances or requirements."
Regards,
-Scott
HKHK
15 Posts
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April 14th, 2016 22:00
Hi Scott,
Totally agree with you . I saw it one of the discussions
https://community.emc.com/message/706522#706522
Regards,
Sutirtha
HKHK
15 Posts
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April 18th, 2016 07:00
Any further points would be great help . Still in confusion about the numbers
Rainer_EMC
8.6K Posts
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April 18th, 2016 07:00
Follow the best practice document and configure as much write cache as possible
The actual max amount possible varies and does depend on which Enablers you have installed
ZaphodB
195 Posts
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April 18th, 2016 07:00
Having never owned the bigger VNX units I have never had reason to be aware of any maximum values.
But I would still stand by my advice to use as much for write cache as possible while providing at least the minimum for read. So if the slider for write really only goes to ~16GB, and that leaves you closer to 4GB for read, that's what I'd do.
If the array were running under load and you were tuning cache I would advise you to provide enough write cache to eliminate, or at least minimize, forced flush activity. If you discovered that you had absolutely no forced flushes you *could* lower the write cache later on; but even doing that would be unlikely to improve read performance to a measurable/significant degree.
HKHK
15 Posts
0
April 19th, 2016 20:00
Thanks .