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[Qemu-devel] Very slow finding extents in QCOW2-backed nbd

From: Tim Smith
Subject: [Qemu-devel] Very slow finding extents in QCOW2-backed nbd
Date: Mon, 28 Jan 2019 11:58:30 +0000

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 <address@hidden>

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