ext3 with maildir++ = huge disk latency and high load

Bob kwijibo at zianet.com
Fri Sep 23 17:19:30 UTC 2011

On 09/22/2011 11:51 PM, Andrey wrote:
> Hello,
> I have a production mail server with maildir++ structure and about 
> 250GB (~10 millions) of files on the ext3 partition on RAID5. It's 
> mounted with noatime option. These mail server is responsible to local 
> delivery and storing mail messages.
> System has Debian Squeeze installed and Exim as MDA + Dovecot as 
> IMAP+POP3 server.
> Bonnie results are terrible. Sequential output for Block and Rewrite 
> are 10722ms and 9232ms. So if there is a 1000 messages in the mail 
> queue load is extremely high, delivery time is very big and server can 
> hang. I did not see such problems with UFS on FreeBSD server.
> As I understand ext3 file system is really bad for such configurations 
> with Maildir++ (many smaill files)? Is there a way to decrease disk 
> latency on ext3 or speed up it?

My guess is that your problem is many files in one directory not necessarily
having many files on the whole file system.  In my experience large 
eat ext3's lunch.  The introduction of indexing did help but it still 
fell behind
on performance when compared to some other file systems.  You may want
to make sure your file system has indexing turned on but with the vintage of
your Debian I would assume it is on by default.

I ran into this problem many years ago (before indexing was an ext3 
option).  It
was even worse as the Maildir storage was being accessed over NFS.  Ended
up eventually biting the bullet and moving to WAFL (NetApp).

My guess is that users trying to access these large directories via IMAP 
and POP
are also facing large delays and possibly even time outs.


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