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Tuesday, June 10, 2014

EMC VNX Default Thresholds Limits and SP Cache Behavior when it reaches beyond the water Mark

Today we had a discussion when one of my customer's VNX storage had performance hit and we have started digging down to find out the issue and finally we came to know it is the unique behavior of FLARE as
it has a condition called Forced Flushing. It occurs when the percent count of dirty cache pages crosses over the high watermark and reaches 100%. At that point, the cache starts forcefully flushing unsaved (dirty) data to disk, suspending all host IO. Forced flushing continues until the percent count of dirty pages recedes below the low watermark.

Forced flushing affects the entire array and all workloads served by the array. It significantly increases the host response time until the number of cache dirty pages falls below the low watermark. The Storage Processor gives priority to writing dirty data to disk, rather than allocating new pages for incoming Host IO. The idea of high and low watermark functionality was implemented as a mechanism to avoid forced flushing. The lower the high watermark, the larger the reserved buffer in the cache, and the smaller chance that forced flushing will occur.

So why "the SP Dirty Page% occasionally can reach 95%"? Because there are too many inbound IOs and the backend disks might have been overloaded thus the cache doesn't have enough time to write them to the disks.

Please also find the Default thresholds for different parameters in EMC VNX

160 iops
2500 iops
70 iops
90 iops
160 iops
120 iops
Dirty pages
95 %
Disk resp. time
>15 ms (if total iops > 20)
Average Seek Distance
10 % or > 30 GB/s
Lun response time
>22ms (if total iops > 20)
BE Bandwidth
>320MB/s or 2160MB/s for VNX

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