From: Thomas Munro <thomas.mu...@gmail.com> > > I'm probably being silly, but can't we avoid the problem by using fstat() > instead of lseek(SEEK_END)? Would they return the same value from the > i-node? > > Amazingly, st_size can disagree with SEEK_END when using the Linux NFS > client, but its behaviour is worse. Here's a sequence from a Linux > NFS client talking to a Linux NFS server with no free space. This > time, I also replaced the fsync() with sleep(60), just to make it > clear that SEEK_END offset can move at any time due to asynchronous > activity in kernel threads:
Thank you for experimenting. That's surely amazing. So, it makes sense for commercial DBMSs and MySQL to preallocate data files... (But IIRC, MySQL has provided an option to allocate a file per table like Postgres relatively recently.) FWIW, it seems safe to use the nodelalloc mount option with ext4 to disable delayed allocation, while xfs doesn't have such an option. > > Or, can't we just try to do BufTableLookup() one block after what > smgrnblocks() returns? > > Unfortunately the problem isn't limited to one block. You're right. The data file can be extended by multiple blocks between disk writes. Regards Takayuki Tsunakawa
