persistent preallocation

persistent preallocation

[Bo Brantén]

Hello,

I have a question on the implementation of persistent preallocation, the 
reason to not return the disk block is because it can contain old data but 
I wonder if an implementation has to return zeros instead or if it could 
return any garbage as long as it is not _that_ garbage?

Bo Branten

Re: persistent preallocation

[Dmitry Monakhov]

Bo Brantén <bosse@acc.umu.se> writes:

> Hello,
>
> I have a question on the implementation of persistent preallocation,
> the reason to not return the disk block is because it can contain old
> data but I wonder if an implementation has to return zeros instead or
Please look at fallocate(2) syscall.
> if it could return any garbage as long as it is not _that_ garbage?
Off course it is not a garbage, it is old user's data which may be
very very useful (old passwords, cookies, and etc)for person who has
time to analyze it :)
>
> Bo Branten
>
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Re: persistent preallocation

[Eric Sandeen]

On 08/03/2010 06:18 AM, Dmitry Monakhov wrote:
> Bo Brantén <bosse@acc.umu.se> writes:
> 
>> Hello,
>>
>> I have a question on the implementation of persistent preallocation,
>> the reason to not return the disk block is because it can contain old
>> data but I wonder if an implementation has to return zeros instead or
> Please look at fallocate(2) syscall.
>> if it could return any garbage as long as it is not _that_ garbage?
> Off course it is not a garbage, it is old user's data which may be
> very very useful (old passwords, cookies, and etc)for person who has
> time to analyze it :)

I think the question was, could the implementation return 0xFF rather
than what is actually on disk (and rather than 0's) when reading a
preallocated block.

You could certainly write it that way, but I don't know why you would
want to...

IOW it wouldn't be posix_fallocate() or fallocate(), it'd be
junk_preallocate() or something.

-Eric

>>
>> Bo Branten
>>
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Re: persistent preallocation

[Ted Ts'o]

On Tue, Aug 03, 2010 at 12:58:21PM +0200, Bo Brantén wrote:
> 
> I have a question on the implementation of persistent preallocation,
> the reason to not return the disk block is because it can contain
> old data but I wonder if an implementation has to return zeros
> instead or if it could return any garbage as long as it is not
> _that_ garbage?

What would be the point?  If we know that we can't return the on-disk
block contents, returning all zero's is much nicer for the application
since it also then looks like and works just like a sparse file.

      	      	   	      	  	- Ted
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