Sizing Aria Pagecache

Last updated on 18 September 2021

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 used for internal temporary tables and, potentially, tables created by the user. 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.

Sizing Aria pagecache
Blixy is determining the proper size for Aria pagecache

Should we really care about Aria?

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

  • Some system tables;
  • Internal temporary tables;
  • Tables explicitly created with ENGINE=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 it’s better to refrain from overoptimising.

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. Not all blocks should be 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 to avoid having 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, at any point in time, the number is high and it decreases suddenly, it means that Aria had to flush pages to make room for new data. This probably means that Aria cache is not big enough.

JIRA tasks related to Aria Pagecache

The MariaDB team has public JIRA projects. We can keep an eye on them to follow the development of new features and bug fixing. If you care about Aria Pagecache, the following tasks are important:

If you make massive use of Aria you may also care about this, but it’s much less likely to be implemented:

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 almost equivalent to Aria Pagecache. The big difference is that, while the Pagecache stores both indexes and data, the Key Cache only stores indexes. For each Aria status variable mentioned in this article, there is an equivalent MyISAM variable 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 nowadays. 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 our MariaDB Health Checks, we check Aria pagecache size is set properly.

Federico

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About Federico Razzoli

Federico is Vettabase Ltd director, and he's an expert database consultant specialised in the MariaDB and MySQL ecosystems.

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