Broken uidlist when using NFS on newer kernels

Alessio Cecchi alessio at
Tue Oct 12 18:39:57 EEST 2021

Hi Jeremy,

I had the same problem as you.

We run an email hosting service with Maildir on NetApp NFS, Dovecot 
Director and Backend servers for POP/IMAP and messagges deliverd via 
dovecot-lda by MXs. After the upgrade from CentOS 6 to CentOS 7 I found 
the same issue as you (on dovecot-uidlist).

After many tests we decided to switch from LDA to LMTP, that was already 
in our roadmap, so read and delivery of messagges is always on the same 
backend. And the problem was solved.

I haven't found any others workarounds.

Swith from LDA to LMTP was not so simple for us since our MX wasn't able 
to talk LMTP but we have write some custom C++ code and was done. You 
should also consider to add some directors since also incoming emails 
will transit from it.

If you would like to talk about how we solve on MXs side I will happy to 
talk with you.


Il 08/10/21 21:01, Jeremy Hanmer ha scritto:
> I know this has been reported in the past, but I think I have some 
> useful new information on the problem. After an OS upgrade from Ubuntu 
> Xenial (4.4.0 kernel) to Ubuntu Focal (5.4.0 kernel) and corresponding 
> upgrade from Dovecot 2.2.27 to, we've started seeing broken 
> uidlist files to an extent that's making larger mail boxes nearly 
> unusable because the file is constantly being regenerated. I've also 
> used the 2.3.16-2+ubuntu20.04 version distributed from 
> <> and the behavior is unchanged. The environment 
> consists of NFS mounts from a NetApp device, with a couple dozen MX 
> servers receiving mail and about a hundred IMAP/POP servers.
> This is the exact error (note the blank after "invalid data"):
> Error: Mailbox INBOX: Broken file 
> /mnt/morty/morty2/gravest/x15775549/Maildir/dovecot-uidlist line 373: 
> Invalid data:
> I've been able to trigger the problem rather easily by piping an email 
> to dovecot-lda in a loop and reading the resulting dovecot-uidlist 
> file on a different server. What it shows is that occasionally we're 
> seeing the last line of the file prepended with a number of null bytes 
> equal to the line that's being written (for example, if the entry is 
> "35322 
> :1633719038.M516419P3623238.pdx1-sub0-mail-mx202,S=2777,W=2832", we'll 
> have it prepended by 69 null bytes). This then breaks the IMAP 
> process' ability to read the file. My first thought was to extend the 
> retry functionality so the imap proces makes more attempts to read the 
> file when it detects a problem like this, but would love input from 
> someone more familiar with the codebase.

Alessio Cecchi
Postmaster @

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