For overwrite case, I checked height of BTree of my lmdb database, Height is 4, so for "one byte" page update, there should be 4 pages update, plus one meta page update, write amplification should be 5 rather than ~9, let me know if I missed something?
by the way, how can I get the degree of B-Tree of lmdb database?
Cheers, xinxin
-----Original Message----- From: Леонид Юрьев [mailto:leo@yuriev.ru] Sent: Tuesday, May 05, 2015 6:16 PM To: Shu, Xinxin Cc: openldap-technical@openldap.org Subject: Re: large write amplification
Hm, ANY change needs a btree-update.
Let have a item key=K, data=A. Then overwrite A to B, so now key=K, data=B.
This is a simply "one byte" change, but a few db-pages need to be cloned and updated: - a page, which contains the data=B and records around. - a page in b-tree, that holds a pointer/reference to a page, which contains data=B and records around. - all "leaf-to-root path in btree" pages, related to a new page in btree, that holds a pointer/reference to a page, which contains data=B and records around. - ... - a new root-pages of mainDB and freeDB. - a point to "new root" in meta-page, that lay in the house that Jack built ;)
So, by design LMDB is optimized for highload reading, but not for writes.
Leonid.
2015-05-05 10:26 GMT+03:00 Shu, Xinxin xinxin.shu@intel.com:
Hi leonid,
Thanks for your reply, I observed another scenario , I also tested "overwrite mode", I slightly modify source code to change default behavior (set dbflags_ = SYNC, flush data to disk once transaction is committed ), also collected iostat , the overwrite IOPS is ~ 521 ops/sec , but iostat show that w/s is ~ 4666, the write amplification is ~9, to my understanding, overwriting exist value does not adjust btree, why write amplification is so large, could you help explain ? thanks
Cheers, xinxin
-----Original Message----- From: Леонид Юрьев [mailto:leo@yuriev.ru] Sent: Monday, May 04, 2015 6:59 PM To: Shu, Xinxin Cc: openldap-technical@openldap.org Subject: Re: large write amplification
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