UBIFS data integrity

[Mats Kärrman]

On Thursday, January 16, 2014 9:16 AM, Artem Bityutskiy wrote:
> > UBIFS writes in 4KiB units.
> 
> I guess this is a bit confusing. The low-level flash write operations
> can be bigger or smaller. However, the file data "buffers" that UBIFS
> writes contain 4KiB of file data (aligned, e.g., regions of the file
> would be 0-4K, 4K-8K, etc). Every such "buffer" is protected with CRC32,
> and if a power cut results in the buffer not being fully written, the
> CRC32 mismatches and UBIFS discards the entire buffer. And the old
> contents of this file region is stays on the flash. The old contents
> does not go away before the new contents reaches the flash media. So the
> read operation for any aligned 4KiB region always returns the old
> contents, or the new contents, nothing else.
> 
> >  So any aligned 4KiB piece of your file is
> > guaranteed to either be fully written, then the read will return new
> > data, or not be written at all, then the read will return old data.

Sorry if I mislead you, I guess the final statement of my original mail is
still valid then:
"In general this requires that your database also is implemented
in a fault tolerant way."
I.e. it must itself make sure it's entire write is completed before
switching to a new "state". Unless the DB implements some kind of
journaling this may be a problem.

BR // Mats