blockdev --flushbufs required [was: parted issue/question
 

Dne 18.1.2012 14:44, Jim Meyering napsal(a):

[Following up on this thread:
http://thread.gmane.org/gmane.linux.kernel.device-mapper.devel/14999]

Alasdair G Kergon wrote:

Try
blkdev --flushbufs
after any cmd that writes to a dev to see if that makes any difference.


Thanks for the work-around.
Using "blockdev --flushbufs $dev" does indeed make parted
behave the same with dm-backed storage as with other devices.

Adjusting my small example,

cd /tmp; truncate -s 10m g&& loop=$(losetup --show -f g)
echo 0 100 linear $loop 0 | dmsetup create zub
dev=/dev/mapper/zub
parted -s $dev
mklabel gpt
mkpart efi 34s 34s
mkpart root 35s 35s
mkpart roo2 36s 36s
u s p
blockdev --flushbufs $dev # FIXME: required with device-mapper-1.02.65-5

# write random bits to p1
dd of=${dev}p1 if=/dev/urandom count=1
dd if=${dev}p1 of=p1-copy.pre count=1
parted -s $dev mkpart p4 37s 37s
blockdev --flushbufs $dev # FIXME: required with device-mapper-1.02.65-5

dd if=${dev}p1 of=p1-copy.post count=1
cmp -l p1-copy.pre p1-copy.post

With that, the "cmp" show no differences.

Does this sound like a problem in device-mapper land,
or in how parted interacts with DM?



Just my wild guess it could be related to wrong assumption that close of
descriptor means automatic flush - this is only true in the case, there is
only one user for descriptor so it would be the last user - but if e.g. the
device is opened more then once, then close doesn't mean flush - so I'd have
assume some application is missing fsync(fd) before doing close(fd).


Zdenek

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