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RE: large write amplification
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
>
>