Re: MySQL 5 : safe update ?

[Joe Emenaker <joe@emenaker.com>]

Frédéric VANNIÈRE wrote:
> Is there compatibilities problems between MySQL 4.1 and MySQL 5.0 ?
The one obstacle that will *prevent* your upgrade from even completing 
is that MySQL 5 doesn't read ISAM (".ISA") files anymore. If you have 
any ISAM files (probably created with versions of MySQL 3 or earlier), 
then you have to convert them to MyISAM or InnoDB. I have a script that 
does this if you need.

When I tried to move to MySQL 5 a few weeks ago, I ran into a little 
snafu with this. The system will let you remove MySQL 4 and install 
MySQL 5 *without* checking to see if you have legacy ISAM tables. When 
the MySQL 5 installer script sees them, however, it refuses to run the 
new daemon. So, you're left with version 5 of the daemon which won't run 
because some of your tables are in the old format, but you have no way 
of converting them to the new format because version 5 doesn't read the 
old format.

The only solution is to downgrade to version 4.... *but* there's a 
problem. The version 5 installer places a file in /var/lib/mysql called 
debian-5.0.flag. When the 4.x installer sees this, it refuses to 
install. *So*... you have to delete that debian-5.0.flag, then 
downgrade, then convert your ISAM tables to MyISAM, and then upgrade again.

Of course, you can avoid all of this by just converting to MyISAMs 
before you even try the upgrade to MySQL 5.
> I need to move 4000 databases from one server to another (dump then 
> import)
You don't have to dump and import (unless you really want to). You can 
actually just copy over your entire /var/lib/mysql to the other server.
> and
> I'm wondering if I can use MySQL 5 on the new server without breaking 
> my customer's
> websites.
There are a variety of subtle changes. You can read about them here:

  http://dev.mysql.com/doc/refman/5.0/en/upgrading-from-4-1.html

Apparently, they have a tool called mysql_upgrade that checks your 
tables for any incompatibility issues.

Lastly, you might want to give a little more attention to the changes to 
the SQL language in version 5, since these could affect some of the SQL 
statements used by your customers in their applications.

- Joe

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