dovecot - Jun 2011 - what to expect from changing index location

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Hello everybody,

I have one thousand virtual users with mdbox mailbox format and 10 GByte
quota. I have noticed some performance problem related to I/O (the
mailbox disk is a 6TB raid1+0 on ISCSI), so I want to put the index
files on a different disk. My actual mail_location is:

mail_location = mdbox:/var/vmail/%-1.1u/%u/mdbox

and I want to switch to

mail_location mdbox:/var/vmail/%-1.1u/%u/mdbox:INDEX=/var/indexes/%-1.1u/%u/

But I cannot figure out a pair of things:

- - do the switch trigger the rebuilding of the index files?

- - can I get rid of all the old index files?

- - how much the index files (no fts squat) can grow?

thanks in advance
bye
davide
- -- 
Dott. Davide Vaghetti
Centro Servizi Informatici Facolta' di Ingegneria
Universita' di Pisa
PGP:
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On 2011-06-28 12:13 PM, Davide Vaghetti wrote:
> mail_location = mdbox:/var/vmail/%-1.1u/%u/mdbox
> 
> and I want to switch to
> 
> mail_location >
mdbox:/var/vmail/%-1.1u/%u/mdbox:INDEX=/var/indexes/%-1.1u/%u/
> 
> But I cannot figure out a pair of things:
> 
> - - do the switch trigger the rebuilding of the index files?
> 
> - - can I get rid of all the old index files?
I'm by no means an expert, but with that many users I think if you did
this in one shot (all indexes being rebuilt simultaneously as users
logged in) your system would slow to a crawl...

I would first rsync the existing indexes over live, then stop dovecot,
do another quick rsync of the indexes, then make the change and restart
dovecot...

That will minimize the impact (rebuilding of indexes)...

-- 

Best regards,

Charles

On 28/06/2011 17:13, Davide Vaghetti wrote:
> I have one thousand virtual users with mdbox mailbox format and 10 GByte
quota. I have noticed some performance problem related to I/O (the mailbox disk
is a 6TB raid1+0 on ISCSI), so I want to put the index files on a different
disk. My actual mail_location is:
>
> mail_location = mdbox:/var/vmail/%-1.1u/%u/mdbox
>
> and I want to switch to
>
> mail_location >
mdbox:/var/vmail/%-1.1u/%u/mdbox:INDEX=/var/indexes/%-1.1u/%u/
>
> But I cannot figure out a pair of things:
>
> - - do the switch trigger the rebuilding of the index files?
!!!!! DANGER, DANGER !!!!!!

Index files cannot be re-generated under mdbox

Go away and read http://wiki2.dovecot.org/MailboxFormat/dbox

"... with dbox the Index files actually contain significant data which 
is held nowhere else. Index files for both *single-dbox* and 
*multi-dbox* contain message flags and keywords. For *multi-dbox*, the 
index file also contains the map_uids which link (via the "map index")
to the actual message data. This data cannot be automatically recreated, 
so it is important that Index files are treated with the same care as 
message data files."

If you don't already know this, then you probably shouldn't even be 
using mdbox.
> - - can I get rid of all the old index files?
NO!
> - - how much the index files (no fts squat) can grow?
First solve your understanding problem with mdbox, then worry about 
details such as this.

Bill