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Re: High load times with mdb

Bill MacAllister wrote:

--On June 25, 2013 10:29:26 AM -0700 Bill MacAllister <whm@stanford.edu>

With the release of Debian 7 (wheezy) I was rebuilding a couple test
systems and was surprised to find that the load times I am seeing for
populating the mdb database with slapd have gone up dramatically.  The
load for a master server that was taking about 10 minutes just took
35 minutes.  The slave is worse.  A normal load time is 20 minutes
and it is at 31 minutes now with an eta of about 2.5 hours.

I tested this file system and that file system with this set of options and
that set of options and really never moved the problem significantly.  I
finally realized I should believe my experiments and think about what else
could be causing the problem I was seeing.

What changed about the same time that I starting building with
wheezy/stable was that I removed a partition option that had been added to
improve performance on our VM farm, i.e. align-at:4k.  Our LDAP servers are
physical servers after all.  Reinstating the parameter resulted in
dramatically faster, and reproducible, load times on any file system I
tried.  We are now using ext4 for our LDAP server farm, well, we are when I
get done rebuilding them.

This problem will be specific to the disk in use.  Looking at the
manufacturers documentation it never really states what the block size it,
but implies it is 512 bytes.  I think that is a lie, I am told most modern
disks lie about their geometry.  In any case, for the disks we are using 4k
alignment works.

Thanks everyone for the suggestions.

Thanks for the followup. Modern hard drives have moved to 4096 byte physical sectors, they advertise 512 byte sectors for compatibility with older OSs. This will probably become an issue more often, although I expect modern Linux tools to be able to operate with actual 4096 byte sectors and make the issue more obvious. There should be a drive option that reports its true sector size, I just don't remember the details at the moment.

  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/