Re: Resolving data_offset and data_len

["Simon 'corecode' Schubert" <corecode@xxxxxxxxxxxx>]

Daniel Lorch wrote:
> > cursor.leaf->base.key is the logical *end* (in bytes) of the extent
> > cursor.leaf->data_len is the length of the extent in bytes
> > cursor.leaf->data_offset is the HAMMER internal offset (see below)
>
> I am not sure I understand extents in HAMMER. Let me try:
>
> Suppose we had a file of 24K. This file would be broken up into two
> parts of 16K and 8K respectively. In HAMMER, these parts are called
> ``data records''. Contiguous data records are called ``extents''
> (contiguous relative to what? to the underlying block device?).

Yes.  They are contiguous on the block device, and also in the file.

> Each data record is stored as a leaf node in the B-tree. To obtain all
> leaf nodes belonging to a file, a ``cursor'' is initalized with the
> following values:
>
>   cursor.key_beg.obj_id     <-- the i-number
>   cursor.key_beg.key        <-- is that a byte offset? relative to what?

that's the offset in the file.

>   cursor.asof               <-- what's that for? history/version?

yes!

>   cursor.key_beg.rec_type = HAMMER_RECTYPE_DATA <-- we want a file
>
> hammer_ip_first() and hammer_ip_next() are used to iterate through all
> leaf nodes.
>
> Is that right? (reference:
> http://www.dragonflybsd.org/presentations/nycbsdcon08/img9.png)

Yes, however there are more details to it, because extents may overlap 
(history!), or not exit (sparse files) or be too long (truncations), so 
special care must be taken.

> 1. Does cursor.leaf->base.key indicate the end of the current data
> record or the *entire file*?

the end of (i.e. first offset after) this extent

> 2. For a given file of 12 bytes, cursor.leaf->data_len gives me 16. What
> am I missing?

extents might be padded or files might be truncated.  you always have to 
keep an eye on the inode length.

cheers
  simon

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