dd question (from man page)

jd1008 jd1008 at gmail.com
Wed Nov 19 23:48:15 UTC 2014

On 11/17/2014 03:03 PM, Cameron Simpson wrote:
> On 17Nov2014 13:22, Rick Stevens <ricks at alldigital.com> wrote:
>> On 11/17/2014 11:28 AM, jd1008 issued this missive:
>>> Man page says:
>>> ...
>>>        fdatasync
>>>               physically write output file data before finishing
>>>        fsync  likewise, but also write metadata
>>> There is no explanation about this, as dd is supposed to be agnostic
>>> about the type of the data.
>>> If a disk is being dd'ed  out to a file and the disk is mounted,
>>> (which is a BAAAAD thing to do), then even there, dd is agnostic
>>> about FS data and FS metadata.
>>> So, what data and metadata is the manpage talking about?
>> I believe it's referring to LVM or BIOS RAID metadata.
> Not at all. dd neither knows nor cares about the underlying disc 
> metaphor (RAID, whatever). These two terms refer to the fdatasync and 
> fsync system calls; see "man 2 fdatasync" and "man 2 fsync". To quote 
> from the fdatasync manual entry:
>    fdatasync() flushes all data buffers of a file to disk (before the 
> system
>    call returns).  It resembles fsync() but is not required to update the
>    metadata such as access time.
> and has some followon discussion.
> Cheers,
> Cameron Simpson <cs at zip.com.au>
> Never attribute to malice that which can be attributed to stupidity.
> - Hanlon's Razor
Thank you Cameron.

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