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5036
September 13th, 2012 13:00
How the Forced flushing occurs? What I meant is, will it happens when the percent dirty pages reaches above the HWM level or at 100% of Write Cache?
How the Forced flushing occurs? What I meant is, will it happens when the percent dirty pages reaches above the HWM level or at 100% of Write Cache?
If it occurs at 100% of Write Cache then what is the benefit of changing Water Mark Levels and reduce number of forced flushing?
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dynamox
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September 13th, 2012 13:00
Forced flushing creates space for new I/Os, though they dramatically affect overall performance. When forced flushing takes place, write I/Os to cache are halted while pages are being flushed. Forced flushing will stop when the percentage of dirty pages reaches the low watermark. Forced flushing will occasionally occur even in a well-designed system. Forced flushing starts when the cache is 100% full of dirty pages.
VNX OE for Block tries to keep the number of dirty pages in cache between the High and Low watermarks. The low watermark sets a minimum amount of write data to maintain in cache. It is also the point at which the VNX stops high water and forced flushing. High water flushing starts when the high watermark is exceeded. In a bursty workload environment, lowering both watermarks will increase the cache’s number of free pages beyond the High Watermark, allowing the system to absorb bursts of write requests without forced flushing. If you notice that forced flushing is occurring for most of your LUNs, decrease the watermarks by 10%. As a rule of thumb, the two watermarks should be kept about 20 percentage points apart.
Vibin1
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September 16th, 2012 22:00
What are the date transfer rate (how much MB/s to disk) of Low and High water mark flushing?
You said """"lowering both watermarks will increase the cache’s number of free pages beyond the High Watermark, allowing the system to absorb bursts of write requests without forced flushing.If you notice that forced flushing is occurring for most of your LUNs, decrease the watermarks by 10%.""""" Could you explian me how this happens?
I guess it increase the free pages by starting Lower and Higher water mark flushing 10% earlier than before. Please correct me if I am wrong.
dynamox
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September 17th, 2012 07:00
Low/High water marks are measured in pages, default page size is 8 KB. You can change low/high water marks on the fly in system properties.
Vibin1
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September 17th, 2012 19:00
If we increas Page size to 16KB will that help us to flush out data immediately and reduce the rate of forced flushing?
dynamox
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September 17th, 2012 20:00
you need to talk to your local USPEED guru to help you with your performance issue. It could be having problems destaging because your backend storage is not sized to handle the workload.
RRR
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September 17th, 2012 23:00
As Dynamox said: lower the LW and HW by 10% and see if that helps. You will have flushing going on more often, but at a lower priority plus you have an extra 10% buffer above the HW to absorb write bursts.
DELL-Sheron G
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September 18th, 2012 00:00
These are all guidelines Vibin, the 20% thing would fit like butter on bread for some requirements, but that wont be the case for all.
You got to test with theses settings and find which suits you the best and stick to it.
But playing with the HW and LW is not the permenant fix, there are a lot of other things we gotta check on why the data are being vomitted out by the cache.This just buys you time.
You gotta see how the back end is, where the data is being moved, what RAID you're sitting on. Howmany disks, are there many HOT luns on the same RAID group.
Regards,
Sheron Godfred
Vibin1
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September 18th, 2012 00:00
Actaully my current setting is 50 - 40. But actually I read somewhere like minimum 20% difference should be maintained between HWM & LWM as per EMC to absorb write bursts from hosts. What I would like to know is should I increase this gap to 20% or let it be on the same. Any performace impact on 10%?
DELL-Sheron G
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September 19th, 2012 23:00
Vibin,
Check this post for further details for "Any performace impact on 10%?"
https://community.emc.com/message/676362#676362