This is explicitly noted in https://doc.dovecot.org/2.4.4/core/plugins/fts_flatcurve.html#fts_flatcurve_...

Substring search is likely increasing your storage size by >5x compared to it being off.

This is a limitation of the xapian library itself - it does not natively support substring search (it is designed for prefix search), so it requires extensive additional indexing to get substring search to work.

michael

On 05/14/2026 2:21 PM MDT Jean-Daniel Beaubien via dovecot <dovecot@dovecot.org> wrote:

Hi everyone,

I am currently trying to update from an old dovecot 2.3 setup with fts-lucene to 2.4.2 with fts-flatcurve.

However, I am seeing a giant jump in the size of the indexes.

For an email account that takes 12GB of storage, I get the following index sizes:

• lucene: 270 MB
• flatcurve: 2400 MB

That is almost a 10x increase in the index size (with flatcurve the indexes represent 20% of the size of the mailbox).

Considering I have a mailbox that takes 270GB, I can't imagine the size of the indexes.

*Am I doing something wrong with flatcurve? How can the indexes be so much larger than lucene?*

I took the default flatcurve configuration from the Ubuntu 26.04 install: mail_plugins { fts = yes fts_flatcurve = yes }

fts_autoindex = yes

language_filters = normalizer-icu snowball stopwords

language_tokenizers = generic email-address language_tokenizer_generic_algorithm = simple

language en { default = yes filters = lowercase snowball english-possessive stopwords }

fts flatcurve {

All of these are optional, and indicate the default values.

They are listed here for documentation purposes; most people should not

need to define/override in their config.

commit_limit = 500

max_term_size = 30

min_term_size = 2

optimize_limit = 10

rotate_count = 5000

rotate_time = 5000

substring_search = yes }

*Any help/insight would be greatly appreciated.*

Best regards,

-J Hi everyone, I am currently trying to update from an old dovecot 2.3 setup with fts-lucene to 2.4.2 with fts-flatcurve. However, I am seeing a giant jump in the size of the indexes. For an email account that takes 12GB of storage, I get the following index sizes:

 o lucene: 270 MB
 o flatcurve: 2400 MB

That is almost a 10x increase in the index size (with flatcurve the indexes represent 20% of the size of the mailbox). Considering I have a mailbox that takes 270GB, I can't imagine the size of the indexes. Am I doing something wrong with flatcurve? How can the indexes be so much larger than lucene? I took the default flatcurve configuration from the Ubuntu 26.04 install: mail_plugins { fts = yes fts_flatcurve = yes }

fts_autoindex = yes

language_filters = normalizer-icu snowball stopwords

language_tokenizers = generic email-address language_tokenizer_generic_algorithm = simple

language en { default = yes filters = lowercase snowball english-possessive stopwords }

fts flatcurve { # All of these are optional, and indicate the default values. # They are listed here for documentation purposes; most people should not # need to define/override in their config. # commit_limit = 500 # max_term_size = 30 # min_term_size = 2 # optimize_limit = 10 # rotate_count = 5000 # rotate_time = 5000 substring_search = yes } Any help/insight would be greatly appreciated. Best regards, -J

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