dovecot - Mar 2017 - SiS hashes file ?

Hello,

In mdbox SiS POSIX mail box setting -

There are hard-links under -

<mail_attachment_dir>/ha/sh/<hash>-<guid-1>
<mail_attachment_dir>/ha/sh/<hash>-<guid-2>

Why is a copy of the file kept under hashes/<hash> ?

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Regards,
Sai Kiran