MySQL InnoDB data file directly on a disk slice (fixed-size)?
First of all, what are the entry types that reside in ibdata1 ? Four(4) entry types:


Table MetaData
Table Data Pages
Index Data Pages
MVCC Data


Whenever an InnoDB table experiences DDL, DML, or being used in a Transaction, all four of these types of entries are either read or written. Meanwhile, if innodb_file_per_table is disabled, all these entry types live in ibdata1. If it is enabled, only the Table MetaData and the MVCC Data would reside in ibdata1 while the Table Data Pages and Index Data Pages would reside in the database subfolder as a .ibd file.

That being considered, what would happen if ibdata1 were placed in another volume and symlinked ?

For starters, how does MySQL represent a table regardless of the storage engine ? As a .frm file. Where do .frm files live ? In the datadir. What's wrong with that ?

Here is an example:

Using the default datadir of /var/lib/mysql, let's use an InnoDB table called mydb.mytable.

With innodb_file_per_table disabled, everything would sit in ibdata1 (which you are proposing to symlink and send off to another data volume). For the table mydb.mytable, this is what you would have:


/var/lib/mysql/mydb/mytable.frm
Everything else about the table lives in ibdata1


Picture this now: You access the table, MySQL would first hit /var/lib/mysql/mydb/mytable.frm and then hit the data and index pages in ibdata1 for mydb.mytable. This would constantly be happening with every access of mydb.mytable. This cascading back-and-forth would somehow make things a little slower and you may not get the performance you were expecting by moving ibdata1 to some other data volume. In fact, the cascading effect would now be a factor of the number of InnoDB tables multiplied by two(2).

Imagine having innodb_file_per_table enabled. Now you would have a slightly different setup:


/var/lib/mysql/mydb/mytable.frm
/var/lib/mysql/mydb/mytable.ibd
MVCC data and Table MetaData would reside in ibdata1


This cascading would be a little worse because the cascading for table access would now occur among three files instead of two.

Here is one more scenario some have thought of: Instead of moving the ibdata1 to a different volume, how about enabling innodb_file_per_table and moving the .ibd files to one or more different data volumes ? The cascading effect would now be a factor of the number of InnoDB tables multiplied by three(3). Percona has expressed very good reasons for not doing this that you will find helpful.