Sizing Aria Pagecache

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I wrote two articles about correctly sizing InnoDB buffer pool: Is InnoDB Buffer Pool big enough? and Can we shrink InnoDB Buffer Pool?.

Aria is a MariaDB storage engine that is always used internally. Its pagecache is the equivalent of InnoDB buffer pool, and this article is about sizing it properly. Note that to adjust the pagecache we need to change the value of aria_pagecache_buffer_size in the configuration file and restart MariaDB.

Sunset on a beach
Cornwall costa

Should we really care about Aria?

All MariaDB users use Aria, but many of them don’t know that. Aria is used for:

  • System tables;
  • Internal temporary tables;
  • Tables explicitly created with Aria.

Rarely people create Aria tables explicitly. For other uses of Aria, the default size of the pagecache is usually big enough. Most of the times it could even be smaller, but try not to overoptimise.

Check your pagecache using the hints in this article. If your pagecache is clearly oversized (say, double of what it should be), consider shrinking it. If it’s undersized, definitely resize it. If you set its size manually, check if your choice was wise.

Checking the size of Aria Pagecache

Here I will describe the status variables that we want to check to verify if the pagecache is correctly sized. How to check them is up to you. I suggest to use a proper monitoring solution to see how these values evolve over time, and their spikes.

Unused blocks

The pagecache is made of blocks. Blocks are not completely used, a portion of them should always be free.

Compare Aria_pagecache_blocks_used with Aria_pagecache_blocks_unused. If unused blocks are close to zero, the pagecache needs to be larger. If unused blocks are consistently a big portion of the total,you can shrink the pagecache.

Pagecache efficiency

The purpose of the pagecache is avoiding a high number of disk operations (reads and writes). If a lot of data are still being read from disk, the pagecache is not big enough. Compare Aria_pagecache_read_requests (the reads requested to Aria) with Aria_pagecache_reads (the number of reads that hit the disk because the desired data was not cached).

Flushed blocks

Look at Aria_pagecache_blocks_not_flushed. If the number is high and it decreases suddenly, it means that Aria had to flush pages to make room for new data. This could mean that Aria cache is not big enough.

About MyISAM

Aria was conceived as a modern, crash-safe MyISAM. These engines are similar in many respects.

MyISAM has a Key Cache, which is basically equivalent to Aria Pagecache. For each Aria status variable mentioned in this article, there is an equivalent MyISAM variables that gives us some information about the Key Cache.

If you use MyISAM on purpose, consider using InnoDB instead. There aren’t many good reasons to choose MyISAM instead. That said, if you use MyISAM for a good reason, check if you can use Aria instead. It’s very possible you can’t, because Aria is sensibly slower on write. But, as a general rule, the least storage engines you use in the same server the better.

See also

Courses

Conclusions

We discussed the methods I know to check if Aria pagecache is reasonably sized. However, don’t be too paranoid about it. You surely should enlarge the pagecache if it is not big enough, but that’s not a common situation. Shrinking it could also make sense to save resources and improve performance, but if the gain is small it’s hardly worth the effort.

As part of MariaDB Health Checks, I check Aria pagecache size is set properly.

Toodle pip,
Federico

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Comments (2)

  1. “If unused blocks are consistently a big portion of the total, the pagecache can be made bigger.”

    Did you mean to say “smaller”?

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