Re: WAL and commit_delay

* Bruce Momjian <pgman@candle.pha.pa.us> [010217 14:50]:
> > * Bruce Momjian <pgman@candle.pha.pa.us> [010217 14:46]:
> > > > Right now the WAL preallocation code (XLogFileInit) is not good enough
> > > > because it does lseek to the 16MB position and then writes 1 byte there.
> > > > On an implementation that supports holes in files (which is most Unixen)
> > > > that doesn't cause physical allocation of the intervening space.  We'd
> > > > have to actually write zeroes into all 16MB to ensure the space is
> > > > allocated ... but that's just a couple more lines of code.
> > > 
> > > Are OS's smart enough to not allocate zero-written blocks?  Do we need
> > > to write non-zeros?
> > I don't believe so.  writing Zeros is valid.  
> 
> The reason I ask is because I know you get zeros when trying to read
> data from a file with holes, so it seems some OS could actually drop
> those blocks from storage.
I've written swap files and such with:

dd if=/dev/zero of=SWAPFILE bs=512 count=204800

and all the blocks are allocated. 

LER

> 
> -- 
>   Bruce Momjian                        |  http://candle.pha.pa.us
>   pgman@candle.pha.pa.us               |  (610) 853-3000
>   +  If your life is a hard drive,     |  830 Blythe Avenue
>   +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026
-- 
Larry Rosenman                     http://www.lerctr.org/~ler
Phone: +1 972-414-9812                 E-Mail: ler@lerctr.org
US Mail: 1905 Steamboat Springs Drive, Garland, TX 75044-6749