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Re: How to calculate maxsize for an MDB [was: 2.4.32: mdb stable enough?]
Peter Marschall wrote:
> Hi,
>
> On Wednesday, 15. August 2012, Howard Chu wrote:
>> The MDB library has not yet been optimized for write performance. I expect
>> that there are still some changes for it coming, because it's clear we can
>> still push it further.
>>
>> But feature-wise back-mdb is complete, and we know of several companies
>> that have tested it heavily for their own purposes and are deploying it in
>> production.
>
> Howard's reassuring answer made me wnt to switch from HDB to MDB too.
>
> But this brought up a question:
> Given an existing HDB database, is there a formula or something to calculate
> the 'maxsize' config option of MDB from the existing information.
In my testing, MDB typically uses about 60% as much space as HDB. Factor in
however much future growth you anticipate and go from there.
As an example, I have a test LDIF that's 558694630 bytes, containing 380836
entries. Looking at info from [m]db_stat, we can compare the number of pages
used for each index:
hdb mdb
branch leaf overflow branch leaf overflow
dn2id 328 8097 0 67 7625 0
id2e 344 249856 59368 263 29681 293169
oc 11 154 0 1 3 0
uid 2487 26392 0 65 10895 0
(page sizes are normalized here; hdb id2entry uses 16K pages while all other
databases use 4K pages)
This is with
index objectclass eq
index uid eq,sub
The dn2id, oc, and uid database formats are logically identical between hdb
and mdb, so the difference in size is due to the difference in BDB and MDB.
The id2entry database in mdb uses a slightly different encoding than hdb, so
there are both library and backend format differences there.
As you can see, the more indexing you use, the bigger the difference between
mdb and hdb.
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/