Re: O_DIRECT question

[Denis Vlasenko <vda.linux@googlemail.com>]

On Thursday 25 January 2007 20:28, Phillip Susi wrote:
> > Ahhh shit, are you saying that fdatasync will wait until writes
> > *by all other processes* to thios file will hit the disk?
> > Is that thue?
> 
> I think all processes yes, but certainly all writes to this file by this 
> process.  That means you have to sync for every write, which means you 
> block.  Blocking stalls the pipeline.

I dont understand you here. Suppose fdatasync() is "do not return until
all cached writes to this file *done by current process* hit the disk
(i.e. cached write data from other concurrent processes is not waited for),
report succes or error code". Then

write(fd_O_DIRECT, buf, sz) - will wait until buf's data hit the disk

write(fd, buf, sz) - potentially will return sooner, but
fdatasync(fd) - will wait until buf's data hit the disk

Looks same to me.

> > If you opened a file and are doing only O_DIRECT writes, you
> > *always* have your written data flushed, by each write().
> > How is it different from writes done using
> > "normal" write() + fdatasync() pairs?
> 
> Because you can do writes async, but not fdatasync ( unless there is an 
> async version I don't know about ).

You mean "You can use aio_write" ?
--
vda