On Wed, Apr 4, 2018 at 12:56 PM, Bruce Momjian <br...@momjian.us> wrote: > There has been a lot of focus in this thread on the workflow: > > write() -> blocks remain in kernel memory -> fsync() -> panic? > > But what happens in this workflow: > > write() -> kernel syncs blocks to storage -> fsync() > > Is fsync() going to see a "kernel syncs blocks to storage" failure? > > There was already discussion that if the fsync() causes the "syncs > blocks to storage", fsync() will only report the failure once, but will > it see any failure in the second workflow? There is indication that a > failed write to storage reports back an error once and clears the dirty > flag, but do we know it keeps things around long enough to report an > error to a future fsync()? > > You would think it does, but I have to ask since our fsync() assumptions > have been wrong for so long.
I believe there were some problems of that nature (with various twists, based on other concurrent activity and possibly different fds), and those problems were fixed by the errseq_t system developed by Jeff Layton in Linux 4.13. Call that "bug #1". The second issues is that the pages are marked clean after the error is reported, so further attempts to fsync() the data (in our case for a new attempt to checkpoint) will be futile but appear successful. Call that "bug #2", with the proviso that some people apparently think it's reasonable behaviour and not a bug. At least there is a plausible workaround for that: namely the nuclear option proposed by Craig. -- Thomas Munro http://www.enterprisedb.com
