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Adjusting HW/LW Cache Settings
I am looking to change our high/low watermark settings from 90/70, respectively to 80/60. Can anyone tell me if there is a best practice procedure for adjusting the high/low watermark cache settings on a CX4 array? As an example, should the low watermark be changed first and the settings applied or can both settings be changed at the same time and then applied?
Thanks,
Tim
Jim_A1
59 Posts
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August 31st, 2010 18:00
Tim, in answer to your question, there is no difference in changing the hi/low watermarks for any array.
you should be able to change the watermarks settings without disrupting anything.
Change the low watermark first, then the high watermark, as the low watermark must always be lower than the high watermark.
Jim
kenn2347
542 Posts
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August 31st, 2010 07:00
The default for a CX4 is 60/80. looks like yours was changed away from that in the past.
I havent seen anything saying to do one at a time. Also changing them might force a flush so the new setting can take effect.
RyanP2
261 Posts
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August 31st, 2010 09:00
Default for CLARiiON arrays in 60/80 EXCEPT when the CLARiiON is behind a Celerra (integrated or gateway). In this case they are usually 70/90. They should be 60/80 and I find changing the low watermark first and they the high watermark works best. If you drop the high below the low watermark the gui will complain.
This change can be done with cache enabled.
-Ryan
tthomas1
10 Posts
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September 1st, 2010 09:00
Thanks for all the input. The changes were made with no problems. I appreciate the help.
Tim
Anonymous
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274.2K Posts
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February 22nd, 2012 01:00
ex. 4GB write cache and watermark values is 60/80,
when fast cache is enable the write cache decreased to 3GB.
What should the watermark lo/hi settings to maintain performance?
Storagesavvy
474 Posts
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February 22nd, 2012 10:00
If you want to keep the same amount (in GB) of free write cache above the high watermark after the write cache size is reduced (from enabling features like FASTCache) you need to calculate the new HW % based on the previous HW% and before/after cache size..
For example…
Before: 4GB cache / 80% HW = 4096 * (1-0.8) = 819.2GB cache above the HW
After: 3GB cache = 1- (819.2 / 3072) = new HW = 73%
Set LW to 20 less than HW, so new LW/HW = 53/73
Note: Keep in mind, when enabling FASTCache there can be significant benefit to the cache performance already, and adjusting the HW/LW may not be necessary. Use analyzer to watch the dirty pages%, as long as you aren’t seeing dirty pages climbing up to 100% you really don’t need to lower the watermarks. Higher watermarks can increase write and read cache hit ratios so it’s best to leave them as high as you can as long as dirty pages aren’t hitting 100% (which causes a forced flushing condition).
Assuming you are not experiencing forced flushing, there is no performance benefit to lowering the watermarks, and actually it could reduce overall performance due to possibly lower cache hit ratios.
Richard J Anderson