cyrus.cache not working for me?

[Daniel Eckl]

Dear list members,

when I connect a new IMAP client to my mailbox, it logically fetches all 
headers of a folder I open.

My problem is, that this initial header fetch takes very long in my 
opinion (nearly 10 minutes for about 6000 mails) and while the client is 
fetching, there is a lot of disk activity, iowait goes up to 90-98% and 
load goes up to sometimes 10-15(!) depending if the server is under more 
normal usage load or idling.

I thought, that the file cyrus.cache keeps all headers of mails there 
were already fetched by a client. So when I have that mailbox in another 
client (e.g. at work and now want to connect from home),
then cyrus imapd should just take the header informations requested from 
the client out of the cyrus.cache file and everything should be fast and 
smooth...

So is my cyrus.cache broken in all my folders? Is the function broken 
which should use this file? Or is there a limit, so cyrus.cache only 
holds the e.g. 300 last fetched message headers?

I really need help here. I have a lot of mailboxes with a lot of mails 
in some folders and Outlook seems to fetch all headers again from time 
to time and I get an overlaoded server once or twice a day...

Thank you very much for any hint or help!

Best regards,
Daniel