Hi Mulyadi, These are the sequence of commands i used..
dd if=/dev/zero of=./fsimage.img bs=1k count=10000/sbin/losetup /dev/loop0 ./fsimage.img/sbin/mke2fs /de/loop0 10000mkdir mntpointmount /dev/loop0 ./mntpointcd mntpointvim newfile/* entered some contents, saved and exited*/cd ..rm fsimage.imgcd mntpoint/* I am now able to change to this directory ! */cat newfile/* the contents are here without any change ! */
Hi Sergei, Thank you for ur explanation.. Please find my doubts inline...
2009/1/4 Sergei Trofimovich <slyfox@xxxxxxxx>:> On Sun, 4 Jan 2009 18:39:10 +0530> "Paraneetharan Chandrasekaran" <paraneetharanc@xxxxxxxxx> wrote:>>> Hi All,>> I created a file of 10 MB from /dev/zero using dd command.>> Then i associated /dev/loop0 to that file using losetup command. Then>> i created an ext2 filesystem above /dev/loop0 using mke2fs command.>> Then i mounted the that loop device in a mount point. I entered the>> monted directiry and created on file and entered some contents to that>> file. Then i came out of the directory. Then i deleted the file i>> associated. Then If i go inside the mounted directory, i was able>> change the directory and the file was there with the contents i put.>>>> how is that possible when deleted the file itself inside which i>> created the ext2 filesystem..? I was previously thinking that the VFS>> reads from the file itself for files and directory info. is it not>> like that..? I have even cleared the buffers after i deleted the>> file(using sync).>> When you unlink file - directory entry(dentry) is deleted from directory> structure immediately (metadata operation). So you can't open deleted file by it's old _name_.>> Inode (structure, holding info about physical blocks belonging to associated file) is> deleted when there is no more references to it.>> References can be:> * on disk (hard links)> * in-memory descriptors (files currently opened by process(es))>> Thus you can safely use code like this:> ...> char tmpl[] = "/tmp/tmp.XXXXXXXX"> int fd = mkstemp (tmpl); /* create inode and dentry */> (assert fd != -1);> unlink(tmpl); /* delete dentry */>> /*> * Do something with fd: read/write> */>> close(fd); /* delete inode */> ...>> Moreover, you can get access to this (deleted) file until process exits or closes file.>> You could play with it (prompt 1 emulates losetup, prompt 2 emulates your actions):> prompt1$ cat > /tmp/z> FOO> BAR> <leave it running>> prompt2$ rm /tmp/z> $ls -l /proc/*/fd/* 2>/dev/null | grep /tmp/z> l-wx------ 1 slyfox slyfox 64 Янв 4 18:39 /proc/22665/fd/1 -> /tmp/z (deleted)> $ cat /proc/22665/fd/1> FOO> BAR>> But!.>> My examples are userspace reference holders and they are visible via /proc/*/fd,> losetup (or mount -o loop), in contrast, increments reference count> to associated file differently - via ioctl() call. Does this mean loop device driver is holding anotherreference in kernel space..?
>> You can observe dark magic by `strace'ing losetup command:>> # strace losetup /dev/loop0 /etc/fstab> ....> open("/etc/fstab", O_RDWR|O_LARGEFILE) = 3> open("/dev/loop0", O_RDWR|O_LARGEFILE) = 4> ....> ioctl(4, 0x4c00, 0x3) = 0 // ahha, that's it! We ask kernel to do something nasty with this descriptor> close(3) = 0> ioctl(4, 0x4c04, 0xbf9c0624) = 0 // and here too> close(4) = 0> ....> /usr/include/linux/loop.h:#define LOOP_SET_FD 0x4C00> /usr/include/linux/loop.h:#define LOOP_SET_STATUS64 0x4C04>> So, your ext image file will be deleted right after you supply losetup -d /dev/loop0 (or umount)> Before this action you can play with /dev/loop .> I found an another observation too. Now i dint delete theimage file.. I opened the image file and searched for the contents iput in that newly created file.. It was there in some corner.. Then iclosed the image file by ":q".. Then i added some more data in thenewly created file... Then i came back to the image file and searchedwhether the newly added contents have reflected here or not.. Ofcourse, the newly added contents and old contents were in that imagefile.. Then i was happy and closed by the image file by ":wq"..
Then i entered into the mounted directory and opened the file icreated.. I again entered some other contents. Then i came back to theimage file and opened it.. The changes are no longer there.. I againand again added new contents in the file inside mount point.. Thechanges were no longer in the image file.. But all the newly addedcontents were not go into the air as i was able to see them in thefile inside the mounted directory forever..
summary: When the image file is closed with ":wq", the chages made inthe file inside mounted dir are no longer reflected. But the changesand new additions are retained in the file inside mountpoint. where dothe contents are coming from..? and why this loop association goesaway when i close it with ":wq" ( writing in the iamge file des)?
Thanks,Paraneetharan C>>>> Thanks in advance,>> Paraneetharan C>>>> -->> Sergei>ЪТХ╨{.nг+┴╥÷╝┴╜├+%┼кЪ╠Ищjw╕j)p┼ьЪ╨{.nг+┴╥╓z╧ч√w╟n'╛Чз q╙М┼оГ≈Ыb·ЛЪ╒╦∙Ф╛z╥ vь^╤m╖ЪЪ▒ЙГzYча╦·ЁЗ+┐Я@
