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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