Re: Re: [ntdev] Re: [ntdev] How to verify file write to a physical hard disk

the write head for rotating media does not stop to read the checksum bits
for the write and verify that the bits all actually got written to the
media correctly, instead the head just moves to the next scheduled sector
and sprays some bits out there, hoping for the best. Almost all the time
this just works, or the fact that it didn’t work is irrelevant.

Unless there is a catastrophic error condition, writes don’t fail. It is
the reads that fail, and they do - that is typically how errors are
discovered on disks.

Mark Roddy

On Wed, Feb 11, 2015 at 7:47 AM, Marion Bond wrote:

> I don’t understand what you are trying to say here.
>
> Sent from Surface Pro
>
> From: Mark Roddy
> Sent: ‎Tuesday‎, ‎February‎ ‎10‎, ‎2015 ‎10‎:‎21‎ ‎PM
> To: Windows System Software Devs Interest List
>
> So, without actually doing the read from media there is no guarantee that
> the write, even with FUA, was successful. The disk hardware on rotating
> media cannot actually wait to test that writes succeeded, the test happens
> when a read fails. You can accommodate that either by paying the cost of
> the read or by replicating the write somewhere else.
>
> Mark Roddy
>
> On Tue, Feb 10, 2015 at 7:15 PM, Marion Bond
> wrote:
>
>> And what is FUA on read useful for? Buggy hardware caches perhaps?
>>
>> Event Windows clustering shared quorum disks on a shared SCSI bus don’t
>> need that
>>
>> FUA on write however, it vital for applications like SQL server or
>> anything else needing ACID properties
>>
>> Sent from Surface Pro
>>
>> From: Peter Wieland
>> Sent: ‎Tuesday‎, ‎February‎ ‎10‎, ‎2015 ‎6‎:‎42‎ ‎PM
>>
>> To: Windows System Software Devs Interest List
>>
>> I think the answer is that it was. I went to look since I couldn’t
>> remember what we used to do, but it doesn’t look classpnp only sets FUA for
>> WRITE requests. There is not an option, short of rolling your own CDBs, to
>> force unit access on a read.
>>
>> -p
>>
>> -----Original Message-----
>> From: xxxxx@lists.osr.com [
>> mailto:xxxxx@lists.osr.com
>> ] On Behalf Of xxxxx@osr.com
>> Sent: Tuesday, February 10, 2015 3:30 PM
>> To: Windows System Software Devs Interest List
>> Subject: RE:[ntdev] How to verify file write to a physical hard disk
>>
>>


>>
>> I traced this once, and it IS set correctly. Or, well, it WAS.
>>
>> Shove a SATA disk on a drive analyzer sometime, and sent it some
>> FILE_FLAG_WRITE_THROUGH reads and writes. You will see FUA set, using
>> standard Windows drivers and a commodity AHCI controller.
>>
>> I can also tell you with certainty that there are
>> disks/controllers/drivers that do not honor the FUA bit correctly. But, as
>> I said in my initial reply, that’s really beyond your ability to control at
>> that point and getting FUA set is the best you can do.
>>
>> Peter
>> OSR
>> @OSRDrivers
>>
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