Error: Mailbox INBOX: file_dotlock_create in directory /var/mail failed: Permission denied

[manmatha]

Hi All,

We are running Round cube on top of dovecot and postfix as our email 
echo system. Main problem we are facing is that, roundcube loads really 
slow for big inboxes (~5000 emails). I found that, while it loads, imap 
process goes high in terms of CPU utilization(~90%). I asked it in some 
roundcube support group, They believe that it is related to dovecot not 
roundcube. Can you shed any light on this issue. Thanks in advance.

Here is $dovecot -n$ output

xyz at roundcube-internal-server-1:/etc/dovecot$ dovecot -n
# 2.3.16 (7e2e900c1a): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.5.16 (09c29328)
# OS: Linux 5.15.0-1024-kvm x86_64 Ubuntu 22.04.1 LTS
# Hostname: roundcube-internal-server-1
auth_mechanisms = plain login
disable_plaintext_auth = no
listen = *
lock_method = dotlock
mail_always_cache_fields = date.received
mail_cache_min_mail_count = 5000
mail_location = 
mbox:~/mail:INBOX=/var/mail/%u:INDEX=/var/indexes/isical.ac.in/%n
mail_privileged_group = mail
mail_sort_max_read_count = 100
mailbox_list_index_include_inbox = yes
mailbox_list_index_very_dirty_syncs = yes
maildir_very_dirty_syncs = yes
mbox_dotlock_change_timeout = 1 mins
mbox_lazy_writes = no
mbox_write_locks = dotlock fcntl
namespace inbox {
   inbox = yes
   location =
   prefix =
}
passdb {
   driver = pam
}
protocols = imap pop3
ssl_cert = </etc/dovecot/private/dovecot.pem
ssl_client_ca_dir = /etc/ssl/certs
ssl_dh = # hidden, use -P to show it
ssl_key = # hidden, use -P to show it
userdb {
   driver = passwd
}
protocol !indexer-worker {
   mail_vsize_bg_after_count = 100
}

On a different note we are getting error occasionally of the following 
type. I dont know if they are related.

Jan 16 15:36:13 roundcube-internal-server-1 dovecot: 
imap(xyz)<361270><aqauu17ycsvAqL8/>: Error: unlink(/var/mail/xyz.lock) 
failed: Permission denied (in file-dotlock.c:195)
Jan 16 15:36:13 roundcube-internal-server-1 dovecot: 
imap(xyz)<361270><aqauu17ycsvAqL8/>: Error: unlink(xyz.lock) failed: 
Permission denied (in file-dotlock.c:195)
Jan 16 15:36:13 roundcube-internal-server-1 dovecot: 
imap(xyz)<361270><aqauu17ycsvAqL8/>: Error: Mailbox INBOX: 
file_dotlock_create(xyz) in directory /var/mail failed: Permission 
denied (euid=10865(xyz) egid=8(mail) missing +w perm: /var/mail, UNIX 
perms appear ok (ACL/MAC wrong?), dir owned by 0:8 mode=0771)
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