Retrieving

[Tobias Reinhard <trtracer@gmail.com>]

Hi,

I want to read the complete CSUM-Tree from userspace. I tried it via the 
ioctl. This is what the code looks like:

struct btrfs_sv2_args sv2_args;
int fd = open(filename, O_RDONLY);
sv2_args.key.tree_id = BTRFS_CSUM_TREE_OBJECTID;
sv2_args.key.min_objectid = 0;
sv2_args.key.max_objectid = -1;
sv2_args.key.min_offset = 0;
sv2_args.key.max_offset = -1;
sv2_args.key.min_transid = 0;
sv2_args.key.max_transid = -1;
sv2_args.key.min_type = BTRFS_CSUM_ITEM_KEY;
sv2_args.key.max_type = BTRFS_CSUM_ITEM_KEY;
sv2_args.key.nr_items = -1;
sv2_args.buf_size = sizeof(sv2_args.buf);
ioctl(fd, BTRFS_IOC_TREE_SEARCH_V2, &sv2_args);

But the device is not small and I hit the limit of the 
btrfs_sv2_args.buf which seems to be 16 MB.

How can I get the *complete* CSUM-Tree?

Limiting to offset does not work (My first idea was to do it this way 
and get it in chunks).

Limiting to a single transid does not work as well because even one 
transid is larger than the limit.

Kernel is 4.15.

Any help would be appreciated.

Best Regards

Tobias