DragonFly hammer List (threaded) for 2009-06
Re: Resolving data_offset and data_len
> 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?).
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
cursor.key_beg.obj_id <-- the i-number
cursor.key_beg.key <-- is that a byte offset? relative to what?
cursor.asof <-- what's that for? history/version?
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
Is that right? (reference:
1. Does cursor.leaf->base.key indicate the end of the current data
record or the *entire file*?
2. For a given file of 12 bytes, cursor.leaf->data_len gives me 16. What
am I missing?
> It is a HAMMER_ZONE_LARGE_DATA offset. Disk offsets in HAMMER are all
> 64bit, but do not directly refer to byte offsets on the disk. Instead
> the offsets have multiple layers (or at least one) of indirections. See
> hammer_disk.h:120 for reference. Basically the right thing to do is use
Thanks, hammer_io_direct_read() is what I was looking for. Now I can
calculate the right offset to locate the file on disk. But I'm still
struggling with the file length.