[Libguestfs] Question about blkdev-setbsz

[Richard W.M. Jones]

> <jzheng> Do you have any idea why blockdev-setbsz succeeds, but does
> not affect output of blockdev-getbsz?

I can reproduce the same issue too:

><fs> blockdev-getbsz /dev/vda 
send_to_daemon: 0xf62050 g->state = 3, n = 40
recv_from_daemon: 0xf62050 g->state = 3, size_rtn = 0x7ffffbb4f00c, buf_rtn = 0x7ffffbb4f000
blockdev --getbsz /dev/vda
proc 60 (blockdev_getbsz) took 0.01 seconds
4096
><fs> blockdev-setbsz /dev/vda 2048
send_to_daemon: 0xf62050 g->state = 3, n = 44
recv_from_daemon: 0xf62050 g->state = 3, size_rtn = 0x7ffffbb4f01c, buf_rtn = 0x7ffffbb4f010
blockdev --setbsz 2048 /dev/vda
proc 61 (blockdev_setbsz) took 0.01 seconds
><fs> blockdev-getbsz /dev/vda 
send_to_daemon: 0xf62050 g->state = 3, n = 40
recv_from_daemon: 0xf62050 g->state = 3, size_rtn = 0x7ffffbb4f00c, buf_rtn = 0x7ffffbb4f000
blockdev --getbsz /dev/vda
proc 60 (blockdev_getbsz) took 0.01 seconds
4096

I checked the source of blockdev, and it is using the BLKBSZSET ioctl.
As far as I can tell if the ioctl returns an error then blockdev
should return an error too (and hence the error should go all the way
out to guestfish).

So it appears that the Linux kernel is not returning an error.

http://lxr.linux.no/#linux+v2.6.35/fs/block_dev.c#L73

The other possibility is that the blockdev --getbsz call is not
returning the block size we just set, but that resolves to this call
inside the kernel:

http://lxr.linux.no/#linux+v2.6.35/include/linux/blkdev.h#L1196

As far as I can see this really ought to work.  Perhaps we need to
perform some other operation between the two calls, like a udev settle
or some sort of rereading block devices ioctl.

If this worries you, please file a BZ.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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