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His presentation "Protecting SW From Itself: Powerfail Atomicity for Block Writes" ( slides) has a section of video where he talks about how power failure impacts in-flight writes on traditional storage. However, it's unclear how you can discover and use any multi-sector guarantee if you aren't currently in a position to send raw NVMe commands.Īndy Rudoff is an engineer who talks about investigations he has done on the topic of write atomicity. Devices that are compliant with that spec have to offer a guarantee of sector write atomicity and may choose to offer contiguous multi-sector atomicity up to a specified limit (see the AWUPF field). The more commonly implemented COMPARE AND WRITE is also atomic (potentially across multiple sectors too) but again it's optional for a SCSI device and comes with different semantics to a plain write.Ĭuriously, the NVMe spec was written in such a way to guarantee sector atomicity thanks to Linux kernel developer Matthew Wilcox. The SCSI disk spec is an example of this and the optional WRITE ATOMIC(16) command can even give a guarantee beyond a sector but being optional it's rarely implemented (and thus rarely used). Sometimes specifications offer atomicity guarantees but only on certain write commands. This is because it would depend on whether the 512 byte sector disk was the one being read by the RAID and how many of the 8 512-byte sectors compromising the 4KiB RAID sector it had written before the power failed. As a thought experiment, you can construct a scenario where each individual disk offers sector atomicity (relative to its own sector size) but where the RAID device does not in the face of power loss. Imagine a RAID 1 array (without a journal) comprised of a disk that offers 512 byte sized sectors but where the other disk offered a 4KiB sized sector thus forcing the RAID to expose a sector size of 4KiB. network attached block devices, certain types of RAID etc.) things are less clear and they may or may not offer sector atomicity guarantees while legally behaving per their given spec. Linux kernel developer Christoph Hellwig mentions this off-hand in the 2017 presentation "Failure-Atomic file updates for Linux"). However, it seems tacitly agreed that non-ancient "real" disks quietly try their best to offer this behaviour (e.g. The traditional (SCSI, ATA) disk protocol specifications don't guarantee that any/every sector write is atomic in the event of sudden power loss (but see below for discussion of the NVMe spec). To clarify, this means if you have a page sized buffer, say 4096 bytes, filled with half Y, half X that we want to keep - and we tell the OS to write that buffer over X, there is no situation short of serious disk failure where the half X that we want to keep is corrupted during the write. That means that if part of X is being overwritten, only the part of X that is being overwritten can be changed, not the part of X we intend to keep. I've been trying to understand the design of ACID systems like databases, and to my naive thinking, it seems firebird, which does not use a write-ahead log, is relying that a given write will not destroy old data (X) - only fail to fully write new data (Y). Can you ever end up with a situation where the data on disk is part X, part Y, and part garbage? With no fancy UPS or battery backed disk controller, you can end up with a torn page, where the data on disk is part X and part Y. Say you have old data X on disk, you write new data Y over it, and a tree falls on the power line during that write. I don't care about what happens in multiple sector writes - torn pages are acceptable. Write of new data succeeds fully or old data is left intact should the power fail immediately following the write command. When the OS sends the command to write a sector to disk is it atomic? i.e.