Hi all, I have a question about the intent of the last call to
bdrv_co_block_status() in bdrv_co_block_status(), in block/io.c about line
2195, which looks like this:
ret2 = bdrv_co_block_status(local_file, want_zero, local_map,
*pnum, &file_pnum, NULL, NULL);
if (ret2 >= 0) {
/* Ignore errors. This is just providing extra information, it
* is useful but not necessary.
*/
I have a large (>2 TB virtual, ~600GB real) QCOW2 file which is being exported
as a network block device, and a connected client has asked for the allocation
information via NBD_CMD_BLOCK_STATUS. The virtual disk is quite fragmented
(deliberatedly; this is a test case), and thus has quite a lot of extents.
Each extent is taking ~0.5s to identify, and pretty much all of that time is
taken up in lseek(SEEK_END) from find_allocation() in block/file-posix.c which
is getting called as a result of the bdrv_co_block_status() mentioned above,
with the result that getting the data out through the nbd is taking an order
of magnitude longer than it should.
I ran some tests with the if-block containing the call removed, and the only
discernable difference was that everything went a lot faster. So I'm wondering
what the intent is for that code, and in what circumstances it is useful?
--
Tim Smith <tim.sm...@citrix.com>
