> -----Original Message----- > From: [email protected] [mailto:rsyslog- > [email protected]] On Behalf Of [email protected] > Sent: Monday, June 08, 2009 6:10 PM > To: rsyslog-users > Subject: Re: [rsyslog] ultra-reliable disk files fsync vs. fdatasync > > On Mon, 8 Jun 2009, Rainer Gerhards wrote: > > > Hi all, > > > > I am working on the ultra-reliable file writing capability. I have revisited > > the doc and am considering fsync() vs. fdatasync(). From the man pages, it > > looks like fdatasync() is sufficient if I just need to ensure that any data > > written can also be read. The advantage of fdatasync() is that it is > somewhat > > faster as not all meta data is necessarily written. > > > > I would appreciate if someone could comment on this issue. > > my understanding is that if you are growing the file you need to do a > fsync of the directory, but if you are re-writing part of an already > allocated file fdatasync is the better thing to use.
The man page[1] is somewhat vague. It tells: "Calling fsync() does not necessarily ensure that the entry in the directory containing the file has also reached disk. For that an explicit fsync() on a file descriptor for the directory is also needed." But also "fdatasync() is similar to fsync(), but does not flush modified metadata unless that metadata is needed in order to allow a subsequent data retrieval to be correctly handled. For example, changes to st_atime or st_mtime (respectively, time of last access and time of last modification; see stat(2)) do not not require flushing because they are not necessary for a subsequent data read to be handled correctly. On the other hand, a change to the file size (st_size, as made by say ftruncate(2)), would require a metadata flush. " So my current understanding is that fdatasync() is always safe, but there still exist situations where a sync on the directory is needed. What I can think of is a newly created file. I have to admit that I am a bit puzzled of what it takes to do the "explicit fsync on the directory". Most importantly, I need to have the file handle open for writing, else the fsync will potentially fail. I don't think this is such an excellent idea on directories.. I am right now working on this, and my search has not brought up many good hints. So what I currently plan to do is an open(dirname, O_RDWR) on the directory and then do an fsync on it later. And I guess even after the fsync() on the directeroy, there potentially is an issue with newly created files inside newly create directories, but I don't think I will address this immediately. But I may be useful/necessary to add a tree walker who actually walks the full path name and calls the fsyncs on all of them... Comments are highly appreciated. Rainer [1] http://linux.die.net/man/2/fsync _______________________________________________ rsyslog mailing list http://lists.adiscon.net/mailman/listinfo/rsyslog http://www.rsyslog.com
