[Dovecot] index performance issues

[Netserve]

1. POP3 users who keep the mails stored in server. POP3 needs to get a
list of all the mails' virtual sizes, which requires reading all the
mails' contents. This could be avoided by adding ,W=<size> to the
maildir filenames (http://wiki.dovecot.org/MailboxFormat/Maildir),
although you can't do that for existing mails without causing their 
UIDs
to change.

>>>

The wiki doesn't explain how to add the W=<size> to the file names, 
the conf refers to
mail_cache_fields =  size.virtual size.physical
Having set that on a test server/pop account, deleted the index and 
UID file and logged in, I don't see any size details in either the 
filenames or the UID list.

It's not clear if the mail_cache_fields are added to both the file 
names and index cache or what?

John