Re: Shared buffers, db transactions commited, and write IO on Solaris

[Erik Jones <erik(at)myemma(dot)com>]

On Mar 30, 2007, at 8:14 AM, Dimitri wrote:

You are right in that the page size constraint is lifted in that

directio cuts out the VM filesystem cache.  However, the Solaris

kernel still issues io ops in terms of its logical block size (which

we have at the default 8K).  It can issue io ops for fragments as

small as 1/8th of the block size, but Postgres issues its io requests

in terms of the block size which means that io ops from Postgres will

be in 8K chunks which is exactly what we see when we look at our

system io stats.  In fact, if any io request is made that isn't a

multiple of 512 bytes (the disk sector size), the file system

switches back to the buffered io.

Oh, yes, of course! yes, you still need to respect multiple of 512

bytes block size on read and write - sorry, I was tired :)

Then it's seems to be true - default XLOG block size is 8K, means for

every even small auto-committed transaction we should write 8K?... Is

there any reason to use so big default block size?...

Probably it may be a good idea to put it as 'initdb' parameter? and

have such value per database server?

I believe it's because that is a pretty normal Unix kernal block size and you want the two to match.

erik jones <erik(at)myemma(dot)com>

software developer

615-296-0838

emma(r)