Write Skew

PostgreSQL is a portable RDBMS targeting POSIX systems (and Windows).  It also makes some assumptions about the operating system and hardware it's running on that are not covered by POSIX, but hold on all typical systems.  For example: we assume that 32 bit aligned integers can be read and written atomically; that is, without any kind of synchronisation, you might read an arbitrarily stale value but you won't see a "torn" value with a mixture of bits from the before and after values of a concurrent write we assume that system calls (or at least IPC-related syscalls) synchronise memory; that is, if you write to shared memory and then signal another process, the other process will then be able to read the value we assume that disk blocks of 512 bytes (or some multiple) are written atomically when preallocated; that is, if you lose power and then come back up, you'll either see the old or the new version of a 512-byte block, and not a mixture of bits from the