Wednesday, November 28, 2007

Cache Page Size

Cache Page Size

Here we are discussing the use of the Cache Page Size. We say that it is the same as saying Cache Block Size. Each “Page” or block in Cache is a fixed size. And, in the Clariion, the entire Cache is the same fixed size. Therefore, we feel that this is one of the areas in Cache where knowing your environment (applications, etc) can make a difference. In the diagram above, we are illustrating the use of Cache with three different applications, Oracle, SQL, and Exchange. Next to the applications is a Block Size. We are using these three applications in this diagram because these seem to be the most common applications people come to class with.

Next to the applications is a default Block Size. Again, we are only using these as examples. You want to verify the applications running on the Clariion and their Block Sizes.

There are four different Page Size Settings in Cache for the Clariion, 2 KB, 4 KB, 8 KB, and 16 KB. Let’s start with the default Clariion Page Size of 8 KB. Again, every “Page” in Cache will be 8 KB in size. If we have an application like Oracle running on this Clariion, and Oracle using a default Block Size of 16 KB, that would mean that every Oracle Block of data to the Clariion would be broken into two separate Pages in Cache. With SQL writing to this 8 KB Page Size, it is a one to one ratio, as it is with Exchange, however, with every Exchange Block of data, there is a 4 KB waste of space per block, which could be filling up Cache more rapidly with this “wasted space.”

The next Page Size down shows a 4 KB Page Size for Cache. The nice thing about this size in Cache is that there is no wasted space. Exchange is still in a 1:1 ratio of blocks. However, SQL now has to split into two separate Cache Pages, and Oracle splits into four separate Cache Pages. The good thing about this size is “No Wasted Space.” The down side to this is now we have to listen to the Oracle and SQL admins complain about performance.

So, we set the Page Size to 16 KB to appease the Oracle and SQL admins. Here comes the problem again of wasted space in cache, which, depending on your Clariion, you don’t have a lot of. With the 16 KB Page Size, all of the applications write to one Cache Page. The applications are happy because of this, but we are back to the wasted space. For every Exchange block written to the Clariion, there is a waste of 12 KB Cache space. For every SQL Block, there is a waste of 8 KB Cache Space.

If you are only using one of these applications on the Clariion, great, match the Cache Page Size to that application. If that is not the case, you as the Storage Administrator, will have to decide the Winners and Losers. Next to each of the different page sizes, we have listed the Winners, and the Losers.

In the 8 KB Page Size, SQL and Exchange are winners because from the application point of view, they are a 1:1 ratio. Oracle is a Loser because it is split across two separate blocks in Cache. Another loser in this setting is the Clariion Cache because of the wasted space.

In the 4 KB Page Size, Exchange and Cache are winners because Exchange is again a ratio of 1:1, and no wasted space in Cache. Oracle and SQL are losers because they are written to separate Pages in Cache.

With the 16 KB Page Size, the applications all win. Oracle, SQL and Exchange are all a 1:1 ratio. The big loser in this setting is Cache. Cache is a loser with all of the wasted space.
This, again is one of the places to look at for performance of Cache in a Clariion. Knowing your environment plays a big piece in how things are written to Cache.


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Jungle said...

very useful 3Q.

does it also apply in SuperSpeed SuperCache Cache Page Size settings?

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owl said...

and how clariion do prefetch?
i thought that if our io size is less than page size we still read at least one cache page and if we detect sequential io than even more cache page. is it right? and how cache organize in next gen. storage like vmax ?

thanks for u post

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