total space = (used quota) + (to be expunged)?

[Andre Felipe Machado]

Hello
After reading some archived list msg [1], and database formats doc (quotas 
topic)[2], I realized that there is not a "high level" cyrus command to display 
total disk/partition space actually used at a Cyrus imap backend.
In order to get exact (approximated when single instance store is enabled) to be 
expunged messages space should a du / df be used ?

df_used_blocks - used_quota = space_to_be_expunged

Is there a better approach (more exact, faster, lower system load)?
What is the correct way to account for the single instance store effect?
df or du -L ?
(du -L)-(du)?

At big partitions (with millions of messages) du may cause a high disk load...
Suggestions?

Regards
Andre Felipe

[1] http://lists.andrew.cmu.edu/pipermail/info-cyrus/2006-November/024314.html
[2] http://cyrusimap.org/docs/cyrus-imapd/2.4.13/internal/database-formats.php

[a] http://www.techforce.com.br/news/linux_blog/cyrus_murder_aggregator_2_3_16_debia
n_lenny_tutorial