dovecot - Mar 2014 - imap: Error: mmap() failed with file ... dovecot.index.cache: Cannot allocate memory

Hi,

since some days (about 10) I get the following error in mail error log many,
many times:

...
dovecot: imap(user at domain.com pid:32769 session:<dszL7lX1xADD/uGI>):
Error: mmap() failed with file
/home/dovecotindex/domain.com/user/mailboxes/Trash
/dovecot.index.cache: Cannot allocate memory
....

It's always the same "dovecot.index.cache" file and only for the
same heavily used account.
The account is currently used from about 10 different clients with imap at the
sime time.

I checked the size of the index cache file and it seems very big:

total 2,7G
-rw------- 1 mailstore mailstore  464 M?r 24 14:36 dovecot.index
-rw------- 1 mailstore mailstore  464 M?r 24 14:36 dovecot.index.backup
-rw------- 1 mailstore mailstore 2,7G M?r 24 14:19 dovecot.index.cache
-rw------- 1 mailstore mailstore  140 M?r 24 14:45 dovecot.index.log
-rw------- 1 mailstore mailstore  89K M?r 24 14:36 dovecot.index.log.2

About 2,7 GB?

To solve the problem temporarily, I removed the index files from the index Trash
folder
and Dovecot initialized an index rebuild. Now the size of the index files are
small:

total 28K
-rw------- 1 mailstore mailstore  512 M?r 24 14:47 dovecot.index
-rw------- 1 mailstore mailstore  20K M?r 24 15:28 dovecot.index.cache
-rw------- 1 mailstore mailstore 1,2K M?r 24 15:28 dovecot.index.log


But why could the index cache file be so big?

Many thanks
Urban