Re: [Qemu-devel] [PATCH] HMP: snapshot_blkdev can not consider //root/sn1 and /root/sn1 as the same file

[lijun <junmuzi@gmail.com>]

On 12/09/2013 08:17 PM, Stefan Hajnoczi wrote:
> On Mon, Dec 09, 2013 at 02:06:21PM +0800, jun muzi wrote:
>> If mount a local file(disk) in two different dirctories, it is similar to the
>> network storage. Detecting identical "files" is still a problem.
>> Such as:
>> dd if=/dev/zero of=aa bs=1M count=10
>> mkfs.ext4 aa
>> Then mount aa to two directories.
>> mount aa /mnt/dir1
>> mount aa /mnt/dir2
> (Off-topic, but you should never mount a regular file system read/write
> more than once at a time.  This example is wrong, it can corrupt the
> file system.)
yes, I should not to mount a regular file more than once at a time. But 
I just want to say the network storage will be mount more than once at a 
time.
Such as :
Create a nfs server, and mount it as followings:
# mount $IP:/home/ /mnt/dir1
# mount $IP:/home/ /mnt/dir2
If create a new file named test in /mnt/dir1, /mnt/dir2 will know this 
new file "test". So the linux kernel can know the file in /mnt/dir1 and 
/mnt/dir2 whether it is the same file or not.
Based on above info, qemu-kvm can know whether file1 in /mnt/dir1 and 
file2 in /mnt/dir2 are the same file or not. How do qemu-kvm distinguish 
between file1 and file2 is the problem we should solve. To solve this 
issue, maybe could refer to the linux kernel. But I don't know now.
----
How to fix this bug:
1, realpath() ---- string-only detection
2, hard/soft link. ----maybe ref 
https://lists.gnu.org/archive/html/qemu-devel/2013-11/msg01840.html
3, distinguish network storage file mounted on different dir is the same 
file or not.
> My question was about the bug.  What is the problem you are seeing and
> how do you reproduce it (including the HMP commands)?
Now I am seeing is how to "distinguish network storage file mounted on 
different dir is the same file or not".
Reproduce this bug using nfs mounted in different directories:
Create a nfs server, and mount it as followings:
# mount $IP:/home/ /mnt/dir1
# mount $IP:/home/ /mnt/dir2
(qemu) snapshot_blkdev drive-scsi0-0-0  /mnt/dir1/sn1
Formatting '/mnt/dir1/sn1', fmt=qcow2 size=32212254720 
backing_file='/home/juli/RHEL-7.0-20131127.1.qcow2_v3' 
backing_fmt='qcow2' encryption=off cluster_size=65536 lazy_refcounts=off
(qemu) snapshot_blkdev drive-scsi0-0-0  /mnt/dir2/sn1
Formatting '/mnt/dir2/sn1', fmt=qcow2 size=32212254720 
backing_file='/mnt/dir1/sn1' backing_fmt='qcow2' encryption=off 
cluster_size=65536 lazy_refcounts=off
(qemu) info block
drive-scsi0-0-0: removable=0 io-status=ok file=/mnt/dir2/sn1 
backing_file=/mnt/dir1/sn1 backing_file_depth=2 ro=0 drv=qcow2 
encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
drive-virtio-disk1: removable=0 io-status=ok 
file=/home/juli/data_disk.qcow2_v3 ro=0 drv=qcow2 encrypted=0 bps=0 
bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
ide1-cd0: removable=1 locked=0 tray-open=0 io-status=ok [not inserted]
floppy0: removable=1 locked=0 tray-open=0 [not inserted]
sd0: removable=1 locked=0 tray-open=0 [not inserted]
(qemu) snapshot_blkdev drive-scsi0-0-0  /mnt/dir2/sn2
Could not open '/mnt/dir2/sn1': Could not set AIO state: Too many open 
files: Too many open files

Best Regards,
Jun Li
>
> Stefan