[Date Prev][Date Next]
Re: large write amplification
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
- 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.
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
2015-05-04 5:31 GMT+03:00 Shu, Xinxin <firstname.lastname@example.org>:
> 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