Comment 14 for bug 55692

I don't see anything out of the ordinary. You have a lot of cache used, but again, that's normal, especially if you are doing a lot of disk related things (like web, sql, etc).

The cache isn't "used memory" per se, but it does show up in the free list as being used. If userspace programs need this memory, the kernel will automatically free from the cache.

I think the memory aspect of this bug report is a non-issue. If you want more assistance with the I/O error, please provide a full dmesg with this problem occuring in it.