[Dovecot] Question about "slow" storage but fast cpus, plenty of ram and dovecot

Eric Rostetter rostetter at mail.utexas.edu
Mon Dec 13 06:11:54 EET 2010

Quoting Javier de Miguel Rodríguez <javierdemiguel at us.es>:

>     - I understand that indexes should go to the fastest storage I  
> own. Somebody talked about storing them in a ramdisk and then backup  
> them to disk on shutdown. I have several questions about that:
>                 - In my setup I have 25.000+ users, almost 7.000.000  
> messages in my maildir. How much memory                 should I  
> need in a ramdisk to hold that?

Are you using dovecot with them now?  If so, then you can figure out who
much they are currently using. Otherwise, well, who knows?  It will
depend on the clients used (for dovecot.index.cache), as
well as how often they are accessed (for transaction log), and so on.

Maybe Timo or someone more into the inner workings can give more details.

>                  - What happens if something fails? I think that if  
> I lose the indexes (ej: kernel crash) the next time I                 
>  boot the system the ramdisk will be empty, so the indexes should be  
> recreated. Am I right?

Yes.  If the ramdisk is full, it will switch to INDEX=MEMORY automatically
for all new sessions until space frees up.  And if you crash without saving
the indexes, it will rebuild them when the users reconnect.

>                 - If I buy a SSD system and export that little and  
> fast storage via iSCSI, does zlib compression                         
>  applies to indexes?

I don't think so, but maybe Timo can say for sure.

>     - Any additional filesystem info? I am using ext3 on RHEL 5.5,  
> in RHEL 5.6 ext4 will be supported. Any performance hint/tuning (I  
> already use noatime, 4k blocksize)?

Only the ext3 commit interval (raising it might lower your I/O on the SAN)
I mentioned earlier...  But of course, it raises your chances of losing
data in a crash (i.e. you could lose more data, since it flushes less often).
But it is a good trade off sometimes (I always raise it on my laptops in
order to cut down on battery usage).

Eric Rostetter
The Department of Physics
The University of Texas at Austin

Go Longhorns!

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