Bart, > The above implies that this parameter will always be equal to the > logical block size. It does not. Being able to write each individual block in an I/O without tearing does not imply that a device can write two blocks as a single atomic operation. > Writes to a single physical block happen atomically. If there are > multiple logical blocks per physical block, the block device must > serialize read/modify/write cycles internally. This is what SBC has to say: "If any write command that is not an atomic write command, does not complete successfully (e.g., the command completed with CHECK CONDITION status, or the command was being processed at the time of a power loss or an incorrect demount of a removable medium), then any data in the logical blocks referenced by the LBAs specified by that command is indeterminate." SBC defines "atomic write command" like this: "An atomic write command performs one or more atomic write operations. The following write commands are atomic write commands: a) WRITE ATOMIC (16) (see 5.48); and b) WRITE ATOMIC (32) (see 5.49)." You will note that none of the regular WRITE commands appear in that list. Now, in practice we obviously rely heavily on the fact that most devices are implemented in a sane fashion which doesn't mess up individual logical blocks on power fail. But the spec does not guarantee this; it is device implementation dependent. And again, we have seen both hard disk drives and SSDs that cause collateral damage to an entire physical block when power is lost at the wrong time. -- Martin K. Petersen Oracle Linux Engineering
