DAX on BTT

Posted May 6, 2016 0:00 UTC (Fri) by neilbrown (subscriber, #359)
In reply to: DAX on BTT by phro
Parent article: DAX on BTT

> Traditional storage, however, never guaranteed sector atomicity

citation needed.

My model of traditional storage includes a ECC for each block. So the options for a read after an aborted write are:
- old data
- new data
- read error (ECC reports an uncorrectable error)

How can you get a torn sector?

DAX on BTT

Posted May 6, 2016 0:15 UTC (Fri) by andresfreund (subscriber, #69562) [Link] (1 responses)

I'd argue that a read error is an atomicity problem.

DAX on BTT

Posted May 8, 2016 13:39 UTC (Sun) by robbe (guest, #16131) [Link]

Is there any basic device that guarantees no errors, ever? (Sure, you can build stacks of redundancy that make them less and less probable.)

FWIW, ECC does not guarantee detection of errors. I don’t know what the distance of the code used for your disk is (are these values universal?), so I can’t tell what the probability of an undetected error is.

DAX on BTT

Posted May 6, 2016 14:50 UTC (Fri) by phro (subscriber, #29295) [Link]

>> Traditional storage, however, never guaranteed sector atomicity

> citation needed.

I suppose "never" is a strong word. What I meant to say was that the SCSI and ATA standards did not say anything about power-fail write atomicity of a single sector. Because they did not standardize it, you cannot rely on it.