[Xen-bugs] [Bug 1430] Failed to start guest with qcow/qcow2 image against c/s 19349

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http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=1430





------- Comment #5 from rstonehouse@xxxxxxxxxxxxxx  2009-06-02 08:00 -------
(As per comment #3) this affected my work-flow as well - so I investigated a
little further.

You can get messages from blktap/tapdisk by altering /etc/syslog.conf to log
debug messages.

The problem is that the empty qcow file is exactly 8K. Unfortunately the size
rounding in this function (for 512 byte aligned direct I/O) rounds 8192 to 8704

xen-unstable.hg/tools/blktap/block-qcow.c
        lseek(fd, 0, SEEK_SET);
        l1_table_block = l1_table_size + s->l1_table_offset;
        l1_table_block = l1_table_block + 512 - (l1_table_block % 512); 
        ret = posix_memalign((void **)&buf2, 4096, l1_table_block);
        if (ret != 0) goto fail;
        if (read(fd, buf2, l1_table_block) != l1_table_block) {
          goto fail;
        }

It can be corrected in the source as
        l1_table_block = l1_table_block + 511 - (l1_table_block % 512); 
OR
        l1_table_block = ROUNDUP(l1_table_block, 512)


A very easy fix is just to pad your qcow file like this:
   dd bs=512 count=0 seek=17 if=/dev/zero of=linux-0.2.qcow


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