Re: Write changed immediately to disk

[Philip Guenther <guenther+ldapsoft@sendmail.com>]

On Mon, 18 Feb 2008, Chris G. Sellers wrote:
> Is this more of an OS issue versus an OpenLDAP issue?   Since OpenLDAP does 
> not have raw access to the physical disk, it depends on the OS to flush it's 
> buffers, so hitting reset on a system like that may cause things to get lost 
> in the cache/buffer for the file system??

That's what the fsync() and fdatasync() system calls are for.  The 
Berkeley DB library uses them in the transaction-commit operation to tell 
the OS that the transaction log data should be forced to permanent storage 
so that durability is guaranteed.  That can be disabled in the DB library 
using lines in the DB_CONFIG file:
	set_flags DB_TXN_NOSYNC
or
	set_flags DB_TXN_WRITE_NOSYNC

or the "dbnosync" option in the slapd.conf, which sets the DB_TXN_NOSYNC 
flag.

If none of those is present, then the problem is the OS or hardware. 
IIRC, many UNIX variants don't *really* guarantee flushing permanent 
storage unless you disable the disk's write-cache...and even then, some 
hardware lies...


Philip Guenther