Re: Postgres as In-Memory Database? - Mailing list pgsql-general

On Sun, Nov 17, 2013 at 4:46 PM, Edson Richter <edsonrichter@hotmail.com> wrote:

 

Yes, those optimizations I was talking about: having database server store transaction log in high speed solid state disks and consider it done while background thread will update data in slower disks... 

There is no reason to wait for fsync in slow disks to guarantee consistency... If database server crashes, then it just need to "redo" log transactions from fast disk into slower data storage and database server is ready to go (I think this is Sybase/MS SQL strategy for years).

Using a nonvolatile write cache for pg_xlog is certainly possible and often done with PostgreSQL.  It is not important that the nonvolatile write cache is fronting for SSD, fronting for HDD is fine as the write cache turns the xlog into pure sequential writes and HDD should not have a problem keeping up.

Cheers,

Jeff

Hum... I agree about the tecnology (SSD x HDD, etc) - but may be I misunderstood, but I have read that to keep always safe data, I must use fsync, and as result every transaction must wait for data to be written in disk before returning as success.
By using the approach I've described you will have fsync (and data will be 100% safe), but transaction is considered success once written in the transaction log that is pure sequencial (and even pre-allocated space, without need to ask OS for new files or new space) - and also no need to wait for slow operations to write data in data pages.

Am I wrong?

Edson