For fun, there is no guarantee in terms of writing a page in what order it is written. SQLite documents that they assume (but cannot verify) that _sector_ writes are linear, but not atomic. https://www.sqlite.org/atomiccommit.html
> If a power failure occurs in the middle of a sector write it might be that part of the sector was modified and another part was left unchanged. The key assumption by SQLite is that if any part of the sector gets changed, then either the first or the last bytes will be changed. So the hardware will never start writing a sector in the middle and work towards the ends. We do not know if this assumption is always true but it seems reasonable.
You are talking several levels higher than that, at the page level (composed of multiple sectors).
Assume that they reside in _different_ physical locations, and are written at different times. That's fun.
> Currently all hard drive/SSD manufacturers guarantee that 512 byte sector writes are atomic. As such, failure to write the 106 byte header is not something we account for in current LMDB releases. Also, failures of this type should result in ECC errors in the disk sector - it should be impossible to successfully read a sector that was written incorrectly in the ways you describe.
Even in extreme cases, the probability of failure to write the leading 128 out of 512 bytes of a sector is nearly nil - even on very old hard drives, before 512-byte sector write guarantees. We would have to go back nearly 30 years to find such a device, e.g.
https://archive.org/details/bitsavers_quantumQuaroductManual...
Page 23, Section 2.1 "No damage or loss of data will occur if power is applied or removed during drive operation, except that data may be lost in the sector being written at the time of power loss."
From the specs on page 15, the data transfer rate to/from the platters is
1.25MB/sec, so the time to write one full sector is 0.4096ms; the time to
write the leading 128 bytes of the sector is thus 1/4 of that: 0.10ms. You
would have to be very very unlucky to have a power failure hit the drive
within this .1ms window of time. Fast-forward to present day and it's simply
not an issue.
Assume 512 sectors ( I know those are rare ), but I don't think that there is any guarantees that 4KB page would be:
* Written atomically * Written in a particular order