Re: [PATCH 0/4] Fiemap, an extent mapping ioctl - round 2

[Andreas Dilger <adilger@sun.com>]

On Jul 03, 2008  10:37 -0400, jim owens wrote:
>> * FIEMAP_EXTENT_NO_DIRECT
>> Direct access to the data in this extent is illegal or will have
>> undefined results.
>
> will confuse and mislead many people.  And as Andreas said,
> the fe_physical and fe_device can be valid without the ability
> to directly access the data.  I suggest we call this:
>
>   FIEMAP_EXTENT_NO_BYPASS
>
> As in "you can't bypass the filesystem" to directly access it.
>
> Based on what Andreas said the NO_DIRECT (NO_BYPASS) means,
> I disagree with these:
>
>> * FIEMAP_EXTENT_NET
>>   - This will also set FIEMAP_EXTENT_NO_DIRECT
>> The data for this extent is not stored in a locally-accessible device.
>>
>> * FIEMAP_EXTENT_DATA_COMPRESSED
>>   - This will also set FIEMAP_EXTENT_NO_DIRECT
>> The data in this extent has been compressed by the file system.
>>
>> * FIEMAP_EXTENT_DATA_ENCRYPTED
>>   - This will also set FIEMAP_EXTENT_NO_DIRECT
>> The data in this extent has been encrypted by the file system.
>
> None of the above are always NO_DIRECT.  Certainly you can
> read the raw compressed/encrypted data (a backup program might)
> and even a netdev might be accessed if you know how.

I don't see that calling this "NO_BYPASS" is significantly different
than calling it "NO_DIRECT".  You can "bypass" that flag just as
easily, the point is that you may get garbage out of it, don't do that.
I don't think anyone writing an application will be seriously confused.


>> * FIEMAP_EXTENT_NOT_ALIGNED
>> Extent offsets and length are not guaranteed to be block aligned.
>
> To be something like:
>
>    FIEMAP_EXTENT_ENCODED

That doesn't make sense either.  "NOT_ALIGNED" just means that it
isn't a full filesystem block, but it isn't necessarily "encoded"
in any way.

>> * FIEMAP_EXTENT_DATA_INLINE
>>   This will also set FIEMAP_EXTENT_NOT_ALIGNED
>> Data is located within a meta data block.
>>
>> * FIEMAP_EXTENT_DATA_TAIL
>>   This will also set FIEMAP_EXTENT_NOT_ALIGNED
>> Data is packed into a block with data from other files.
>
> With FIEMAP_EXTENT_ENCODED being the top flag set for
> FIEMAP_EXTENT_NET, FIEMAP_EXTENT_DATA_COMPRESSED,
> FIEMAP_EXTENT_DATA_ENCRYPTED, FIEMAP_EXTENT_DATA_INLINE,
> and FIEMAP_EXTENT_DATA_TAIL.  Then FIEMAP_EXTENT_NO_BYPASS
> may or may not also be set and would say "can I get to
> the physical data", and separately "do I need to do
> special processing on it" would be FIEMAP_EXTENT_ENCODED.

It seems to me that "ENCODED" has no relation to "NET" or "DATA_TAIL"
or "DATA_INLINE".

>> * FIEMAP_EXTENT_UNWRITTEN
>> Unwritten extent - the extent is allocated but it's data has not been
>> initialized.  This indicates the extent's data will be all zero.
>
> This should say "will be all zero if read through the filesystem
> but the contents are undefined if read directly."

That does make sense and should be updated.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.