Thank you!! On Wed, Jan 23, 2019, 2:20 PM Heikki Linnakangas <hlinn...@iki.fi wrote:
> On 23/01/2019 01:03, maayan mordehai wrote: > > hello, > > > > I'm Maayan, I'm in a DBA team that uses postgresql. > > I saw in the documentation on wals: > > https://www.postgresql.org/docs/10/wal-intro.html > > In the tip box that, it's better not to use a journaling filesystem. > and I > > wanted to ask how it works? > > can't we get corruption that we can't recover from? > > I mean what if postgres in the middle of a write to a wal and there is a > > crash, and it didn't finish. > > I'm assuming it will detect it when we will start postgres and write that > > it was rolled back, am I right? > > Yep, any half-written transactions will be rolled back. > > > and how does it work in the data level? if some of the 8k block is > written > > but not all of it, and then there is a crash, how postgres deals with it? > > The first time a block is modified after a checkpoint, a copy of the > block is written to the WAL. At crash recovery, the block is restored > from the WAL. This mechanism is called "full page writes". > > The WAL works just like the journal in a journaling filesystem. That's > why it's not necessary to have journaling at the filesystem level. > > - Heikki >
