I lost a picture of Bob Becks ass this same exact way. -B
On Tue, Nov 10, 2009 at 1:29 AM, Nick Guenther <kou...@gmail.com> wrote: > So, as nicely summarized at > http://www.h-online.com/open/news/item/Possible-data-loss-in-Ext4-740467.html, > ext4 is kind of broken. It won't honor fsync and, as a /feature/, will > wait up to two minutes to write out data, leading to lots of files > emptied to the great bitbucket in the sky if the machine goes down in > that period. Why is this relevant to OpenBSD? Well sometimes I've been > writing a file in vi or mg and had my machine go down, and when it > comes back I find that the file is empty and I'm just trying to figure > out if this is just because the data wasn't fsync'd or if it's because > of softdep or what. > > I know this is kind of a newbish question but I have no idea how I'd > go about researching it. And I'd like to sort this out because it's a > big gap in my knowledge. I thought there was a paper on softdep but > http://openbsd.org/papers doesn't have it. > > NetBSD's summary <http://www.netbsd.org/docs/misc/#softdep-impact> says: > "The FFS takes care to correctly order all metadata operations, as > well as to ensure that all metadata operations precede operations on > the data to which they refer, so that the file system may be > guaranteed to be recoverable after a crash. The last N seconds of file > data may not be recoverable, where N is the syncer interval, but the > file system metadata will be. N is usually 30." > > So my interpretation of this is that my missing file is a > to-be-expected ancient part of posix, unless I run sync after every > write. Is this right? Out of curiousity, what would happen if I ran > sync and pulled the power at the same time (that is, what cases can > cause the filesystem to get inconsistent)? > > > But I still don't get how softdeps fits into all this. That page goes on: > > "With softdeps running, you've got almost the same guarantee. With > softdeps, you have the guarantee that you will get a consistent > snapshot of the file system as it was at some particular point in time > before the crash. So you don't know, as you did without softdeps, > that, for example, if you did an atomic operation such as a rename of > a lock file, the lock file will actually be there; but you do know > that the directory it was in won't be trashed and you do know that > ordering dependencies between that atomic operation and future atomic > operations will have been preserved, so if you are depending on atomic > operations to control, say, some database-like process (e.g. writing > mail spool files in batches, gathering data from a transaction system, > etc.) you can safely start back up where you appear to have left off." > > but while I kind of grasp the details, I can't seem to figure out what > they mean in context. > > Enlightenment appreciated! I don't want to be that guy in 20 years who > rewrites the filesystem to be more efficient by not actually writing > the quantum-light-platter. > > (and btw, why isn't http://openbsd.org/papers linked from the front page?) > > -Nick
