mysql - Is it safe to use up all memory on linux server, not leaving anything for the cache?

I have a CentOS server fully dedicated to MySQL 5.5 (with innodb tables mostly). Server has 32 GB RAM, SSD disks, and avarage memory usage looks like this:

So about 25GB is in use and about 6.5GB is cached. I am experiencing performance problems with WRITE queries, so I was thinking, is this the optimal cache size? I might increase innodb buffer size, so that linux cache would become smaller, or decrease it, so it would be bigger.

What is the optimal used/cached memory balance for busy MySQL server on linux?