Cache Allocation

Cache Allocation In the illustration above, we are seeing again that if data is written to one Storage Processor, it is MIRRORed to the other Storage Processor.

A host that writes data to SP A, will mirror to SP B, and vice versa. So, you will be losing some Cache space to this mirroring. In this example, we are setting SP A’s Write Cache to 1 ½ GB. Which means that over on SP B, 1 ½ GB of Cache space will be taken for the Mirroring of SP A’s Write Cache. The same scenario is set for SP B. The same values are transferred across SPs for Write Cache.

SP UsageSP Usage is pre-allocated Cache Space that is used by the Clariion for things like pointers/deltas, SnapView, MirrorView. The amount of space that is lost per Storage Processor for SP Usage depends on a couple of things. First, is the type of Clariion you have. Second, what Flare Code you are running on the Clariion. We’ll talk later where to find the Flare Code your Clariion is running.

In this example, we are using 750 MB per Storage Processor as the vaule for SP Usage. To give you some real numbers: Type of Clariion Flare Code SP Usage:CX3-80 26 1464 MBCX3-80 24 1464 MB

CX700 26 884 MBCX700 24 832 MB

After Write Cache is allocated and SP Usage is taken into account, this leaves us with 250 MB of Cache for Reads.

The nice thing about the Clariion though is that it allows you to change those cache values. Let’s say for instance, that this initial setup above works for you in the mornings when people are writing to a database, but later in the day, the database has more reads. You can take from Write Cache and give the rest to Read Cache. The other nice thing about it is that it can be scripted from the Command Line Interface. Below the chart are the three commands that you can use to change cache.

Command One Before we can change the values of Cache, we must first disable Cache. This command is the command to disable Write Cache, Read Cache of SP A and SP B. Not only does this disable Cache, it also forces a Flush of Cache to disk. This means that the command prompt will not return immediately. There will be a delay in the command prompt returning until Cache is flushed. As I always say, I cannot give you an amount of time that this will take (two weeks). The answer is going to be….”it depends, you’ll have to test it.”

Command Two This is the actual setting of Cache command. By default, the setting of Cache is allocated in MegaBytes. By setting Write Cache to 2048 MB (2 GB), we are telling the Clariion to take that number, and divide half of it for SP A Write Cache, and half for SP B Write Cache. We don’t calculate into this the Mirroring of Write Cache, just the actual usable space. Next, we specify the amount for the Read Cache Size of SP A of 1250 MB (1.25 GB) and the Read Cache Size of SP B of 1250 MB (1.25 GB). Read Caching is not Mirrored, so we must specify both SPs Read Cache. Notice how by simply taking ½ GB away from SP A and SP B Write Cache, we can allocate 1 GB more of Cache space to the SPs for Reads.

Command Three Finally, we have to re-enable Cache. The ones (1) next to –wc, -rca, and –rcb stand for Enabling.

Changing the values of Cache could be done at any time, all day long if you want to, though I wouldn’t recommend it. But, it could prove to be extremely beneficial to performance of the Clariion. Acknowledgements from Writes, and Reading from Cache is going to happen in Nanoseconds as opposed to milliseconds coming from disk.

Another example of why to change Cache could be when Backups are going to occur. Since you will be reading data from Clariion Luns, you could allocate as much Cache to Reads as possible so that the Backup Host could be retrieving data from Cache rather than disk. When the Backups are complete, you could script that the Cache values go back to Production Levels.

source: http://clariionblogs.blogspot.com/2007/11/cache-allocation.html