Greg Smith wrote: > Bruce Momjian wrote: > > xlogdefs.h says: > > > > /* > > * Because O_DIRECT bypasses the kernel buffers, and because we never > > * read those buffers except during crash recovery, it is a win to use > > * it in all cases where we sync on each write(). We could allow O_DIRECT > > * with fsync(), but because skipping the kernel buffer forces writes out > > * quickly, it seems best just to use it for O_SYNC. It is hard to imagine > > * how fsync() could be a win for O_DIRECT compared to O_SYNC and O_DIRECT. > > * Also, O_DIRECT is never enough to force data to the drives, it merely > > * tries to bypass the kernel cache, so we still need O_SYNC or fsync(). > > */ > > > > This seems wrong because fsync() can win if there are two writes before > > the sync call. Can kernels not issue fsync() if the write was O_DIRECT? > > If that is the cause, we should document it. > > > > The comment does look busted, because you did imagine exactly a case > where they might be combined. The only incompatibility that I'm aware > of is that O_DIRECT requires reads and writes to be aligned properly, so > you can't use it in random application code unless it's aware of that. > O_DIRECT and fsync are compatible; for example, MySQL allows combining > the two: http://dev.mysql.com/doc/refman/5.1/en/innodb-parameters.html
C comment updated in git head: * Because O_DIRECT bypasses the kernel buffers, and because we never * read those buffers except during crash recovery or if wal_level != minimal, * it is a win to use it in all cases where we sync on each write(). We could * allow O_DIRECT with fsync(), but it is unclear if fsync() could process * writes not buffered in the kernel. Also, O_DIRECT is never enough to force * data to the drives, it merely tries to bypass the kernel cache, so we still * need O_SYNC/O_DSYNC. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
