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RE: large write amplification
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?
From: Леонид Юрьев [mailto:firstname.lastname@example.org]
Sent: Tuesday, May 05, 2015 6:16 PM
To: Shu, Xinxin
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.
2015-05-05 10:26 GMT+03:00 Shu, Xinxin <email@example.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 ?
> -----Original Message-----
> From: Леонид Юрьев [mailto:firstname.lastname@example.org]
> Sent: Monday, May 04, 2015 6:59 PM
> To: Shu, Xinxin
> Cc: email@example.com
> 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.
> 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 <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