Full_Name: Luca Scamoni
Submission from: (NULL) (126.96.36.199)
Investigating alias dereferencing slowness on a customer production system I
came upon a suspect behaviour with back-hdb/bdb.
It seems that slapd, when dereferencing, instead of looking for the exact
aliased DN, starts scanning more entries than needed (at least this is what I
can tell looking to the logs set to log anything)
On the customer system, with over 700,000 entries, the time taken to perform a
simple search with alias dereferencing for a single entry, takes as much as
performing a full ldapsearch of the entire ldap tree.
To reproduce I created a small testcase (that I can't upload because there's no
space left on ftp.openldap.org/incoming
). The test case contains also the log
and the operations performed on test data to show the issue.
I tested with openldap 2.4.23, 2.4.31 and 2.4.35. The issue is present in all
versions but seems to manifest itself only with back-bdb/hdb. Back-mdb seems not
to show the issue (even looking at the logs) but right now, moving to back-mdb
is not something the customer is ready to take.
I would not recommend switching to back-mdb at this time, see ITS#7577. Alias
dereferencing is broken in a few cases in back-mdb.
Meanwhile, in back-bdb/hdb, it also depends on whether you're searching with
Deref set to Finding, Searching or Always. Deref/Finding works in all 3
backends (bdb/hdb/mdb). (Deref/Searching in back-mdb was mainly broken by the
patches in ITS#7473.)
When Deref is set to Searching/Always, the backend must find every alias
within the target search scope and dereference them. This deref can
potentially expand the scope of the search, e.g. if the alias points to a
completely different subtree. If you have a lot of aliases in the tree, this
is likely going to be very slow.
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/