Clariion Cache Flushing is a process – technique – method that controls when data from cache needs to be flushed out to Disk using patented algorithms that are defined as Watermarking levels.

To set watermarking limits on your Clariion frames, open Navisphere Manager. Once you select the Clariion that you need to set the Watermarking on, right click to select the properties of the Array and go to the Cache tab.

There are several parameters you need to setup in there, without the actual knowledge and understanding of watermarking, please leave the default values in place. You will have options to setup Low Watermark (LWM) and High Watermark (HWM).

Ideally there are three different cache flushing techniques.

Idle Cache Flushing

Watermark Cache Flushing

Forced Cache Flushing

All the three processes and applicable scenarios are defined below.

Idle Cache Flushing

When a host is writing data to the connected Clariion Disk via cache on the Clariion, the Clariion takes that data, writes it to cache and acknowledges back to the host that the data has been written to disk. This data can actually be sitting in the cache or being written to the disk when this acknowledgement goes out. The process happens in 64 Kilobyte chunks when the data is being transferred to the disk from the cache. Again the process of emptying cache and pushing it out to the Disk is called Flushing.

Due to large chunks of data coming in from the host, sometimes Idle Cache Flushing is not able to maintain the Low Watermark (LWM), in those cases Watermark Cache Flushing kicks in.

Watermark Cache Flushing

As you setup your Watermarks using the above process from Navisphere Manager, let’s assume your Low Watermark (LWM) is set at 60% and your High Watermark (HWM) is at 80%. In this scenario, Clariion Algorithms will try to keep your cache levels between 60% and 80% since those are defined as the low and high watermarks.

If for some reason the cache exceeds 80% occupancy (HWM), Forced Flushing kicks in disabling all the write cache in the Clariion.

Forced Cache Flushing

So when both the above Idle Cache Flushing and Watermark Cache Flushing fails, Forced Cache Flushing kicks in.

With Forced Cache Flushing, Write Cache on the Clariion is disabled; a destaging of Cache happens to the disk, no data is written to cache at this time, all data is transferred over to disk so the acknowledgement back to the host after write verification takes longer causing performance issues. Data is queued since cache gets more priority to read / write to disk. The host will starve of I/O during these times. This Forced Flushing phenomenon can take between milliseconds to several seconds depending on the amount of data to be flushed.

This process continues until the LWM is reached which is 60% and then the write caching is enabled again.

The above processes are always happening as your Clariion is serving data in terms of read and write to the attached host systems.

Related posts:

1. All About EMC Clariion CX4
2. Clariion Basics: DAE, DPE, SPE, Drives, Naming Conventions and Backend Architecture
3. Clariion CX Model Comparison
4. Clariion CX Hardware Components
5. Clariion CX3 Models and Key Information