On 03/14/2012 08:05 AM, Oliver Francke wrote:
Hey,
anybody out there who could explain the structure of a rbd-header? After
last crash we have about 10 images with a:
2012-03-14 15:22:47.998790 7f45a61e3760 librbd: Error reading
header: 2 No such file or directory
error opening image vm-266-disk-1.rbd: 2 No such file or directory
... error?
I understand the "rb.x.y"-prefix, the 2 ^ 16hex as block-size. But
the size/count encoding is not intuitive ;)
The data structure is rbd_obj_header_ondisk, defined in
src/include/rbd_types.h.
The size of the objects is stored as a shift value in the 'order' field.
That is, object size is (1 << order) bytes, and the default of 4MB is
order 22. The total size (image_size) is just a number of bytes.
The encoding of snapshots is a bit more painful, since you'd need to
look up the right snapshot ids for each image by looking at its existing
objects. If you don't mind losing the snapshots, you can just zero out
the fields after image_size. Extra zero bytes shouldn't matter after
snap_names_len is 0.
Besides one file, where I "created" a header and putted it via "rados
put" back into the pool, and got some files
back, many of the other images with lost headers have different sizes.
If you don't know the correct size, setting it too high won't use any
more space unless the fs using it is expanded.
We got bad luck again, too many crashed VM's, too much data-loss...
Comments welcome ;)
Oliver.
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