All I want to do is simulate a delay or flakey device. Mount it. And see
how regular read/write commands
such as cat, echo, vim etc behave.
* I created a delay device d0 and mounted it on /mnt
* Copied few files into /mnt
* Invoked dd command on /dev/mapper/d0 and it was slow
* Invoked cat command on some file in /mnt but it was not slow
What is wrong here?
Steps I followed are shown below:
==========================================================
root@nodea:/users/ydev# dmsetup remove_all
#Created a delay device d0
root@nodea:/users/ydev# dmsetup create d0 --table="0 `blockdev --getsize
/dev/sdb` delay /dev/sdb 0 500"
#Put a filesystem on top of d0
root@nodea:/users/ydev# mkfs -t ext3 /dev/mapper/d0
mke2fs 1.41.11 (14-Mar-2010)
Filesystem label=
OS type: Linux
....
....
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done
....
#Mounted the device-mapper device on /mnt.
root@nodea:/users/ydev# mount -t ext3 /dev/mapper/d0 /mnt
#Copied some arbitrary files into /mnt
root@nodea:/users/ydev# cp -r stap_tests /mnt
#Invoked a dd command to check if it is slowing down and it did as seen
below.
root@nodea:/# dd if=/dev/mapper/d0 of=/dev/null bs=400k count=1
1+0 records in
1+0 records out
409600 bytes (410 kB) copied, 1.01523 s, 403 kB/s
#But it did not slow down cat command as seen below.
root@nodea:/mnt/stap_tests# sysctl vm.drop_caches=3
vm.drop_caches = 3
root@nodea:/mnt/stap_tests# time cat 12M > /dev/null
real 0m0.034s
user 0m0.000s
sys 0m0.004s
==========================================================
Thanks,
Yathi
On 10/21/2011 9:21 AM, Bryn M. Reeves wrote:
On 10/21/2011 04:13 PM, Yathindra wrote:
>mount -t ext3 /dev/sdb /mnt
Copied some data into /mnt
>dmsetup create d0 --table="0 `blockdev --getsize /dev/sdb` delay
/dev/sdb 0 500"
device-mapper: reload ioctl failed: Invalid argument
Command failed
You can't create a mapped device on something while it is mounted as
device-mapper needs to open it exclusively.
But as seen from above, dmsetup is failing to create a flakey/delay
device when /dev/sdb
has a filesystem on top of it. Is there any workaround ?
Set up the file system the way you want it on sdb first. Then unmount
it and create the device-mapper devices and mount that.
Regards,
Bryn.
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