Shu, Xinxin wrote:
I tried 1024 & 2048 bytes value size , but the space amplification
is
~2, so If I want to reduce this space amplification, what's your suggestion?
Have no good ideas at the moment. This is inherent to any DB design that
stores metadata co-resident with the data; e.g. same situation exists
with BerkeleyDB. If you can compact your data so that it fits in 4080
bytes, or can split it apart (if it also consists of a header and actual
data), that would work.
Cheers,
xinxin
-----Original Message-----
From: Howard Chu [mailto:[email protected]]
Sent: Wednesday, April 22, 2015 3:48 AM
To: Shu, Xinxin; [email protected]
Subject: Re: LMDB space amplification
Shu, Xinxin wrote:
Hi list,
Recently I have integrated lmdb into ceph , got intial results , I
dump all records of lmdb, there are several different size key-value
pairs,
1. key size : 45 byte, value size = 124 byte 2. key size : 85 byte,
value size = 1187 byte 3. key size : 57 byte, value size = 135 byte
4. key size : 64 byte, value size = 182 byte 5. key size : 33 byte,
value size = 4096 byte (used for real data, about 80 %)
this db contains about ~ 10000000 records, about 80 percent of records are the
last key-value pairs (key size 33 byte, value size 4096 byte), I calculate size
of all key-value pairs, about 33GB , but on-disk size is about 63GB, so space
amplification is ~ 2, is this amplification reasonable ? In which situation can
lmdb get such large space amplification? How can I reduce this amplification?
If you need any other details , please email me , any tips will be appreciate.
LMDB pages have a 16 byte header. In a system with 4096 byte pages, only
4080 bytes are available for user data, so storing a data value of 4096 bytes
will use 2 pages.
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
