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To: =?utf-8?Q?Cl=C3=A1udio?= Martins <ctpm@ist.utl.pt>
Cc: James Bottomley <James.Bottomley@suse.de>,
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Subject: Re: ATA 4 KiB sector issues.
From: "Martin K. Petersen" <martin.petersen@oracle.com>
Organization: Oracle
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Date: Mon, 08 Mar 2010 16:07:14 -0500
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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