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SubjectRe: ATA 4 KiB sector issues.
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>>>>> "Cláudio" == Cláudio Martins <ctpm@ist.utl.pt> writes:

Cláudio> So the question is: what are hard drive makers guaranteeing (if
Cláudio> anything at all)?

No guarantees. Nothing that you can get in writing, anyway.


Cláudio> Was a 512B sector write really atomic?

Sometimes.


Cláudio> Is a 4k one?

Sometimes, maybe.

The problem with 4KB physical blocks is that if you do a partial or
misaligned write you'll end up having to do read-modify-write. And that
introduces are scenario where a subsequent write error will affect
logical blocks that were not part of the I/O request.

However, you also have that with regular drives because they often write
more than the actual block undergoing I/O. For instance to reduce
hotspot bleed to adjacent sectors.

There have been several unsuccessful attempts at nudging the drive
vendors into giving us real guarantees (supercapacitors, NVRAM or
flash-backed write cache). No luck so far. So people that care use
arrays with non-volatile caches.

--
Martin K. Petersen Oracle Linux Engineering
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