Hi, Xinxin.
I will try to answer briefly, without a details:
- To allow readers be never blocked by a writer, LMDB provides a snapshot of data, indexes and directory for each completed transaction.
- Most of a db-pages (which is not changed by a particular transaction) are "shared" between such snapshots. But any changes of data itself and reflection to btree-indexes (include a particular table, free-db, main-db and so forth) require a new pages to be used and written to the disk.
- In a large db a small "one-byte" change may make "dirty" a lot of db-pages (usualy 4K each). For example, one add/del/mod operation in LDAP-db with size of few GB, requires about 50-100 page-level IOPS.
Leonid.
P.S. For highload uses-cases I made a few changes in our fork of OpenLDAP/LMDB. A one of these features we called "LIFO reclaiming". It give us 10-50 times performance boost, especially by engaging benefits of write-back cache of storage subsystem. Nowadays we used it in our production (telco) environment. But currently ones is not safe for all cases, see https://github.com/ReOpen/ReOpenLDAP/issues/2 and https://github.com/ReOpen/ReOpenLDAP/issues/1.
2015-05-04 5:31 GMT+03:00 Shu, Xinxin xinxin.shu@intel.com:
Hi list,
Recently I run micro tests on LMDB on DC3700 (200GB), I use bench code https://github.com/hyc/leveldb/tree/benches , I tested fillrandsync mode and collected iostat data, found that write amplification is large For fillrandsync case:
IOPS : 1020 ops/sec
Iostat data shows that w/s on that SSD is 8093, and avgqu-sz is ~ 1, await time is about 0.16 ms, so the write amplification is ~8, which is large to me, can someone help explain why write amplification is so large? thanks
Cheers, xinxin