Date: Sun, 2 May 2004 00:41:56 -0600 (MDT) From: Siddharth Aggarwal <saggarwa@cs.utah.edu> To: freebsd-fs@freebsd.org Subject: Debugging pseudo-disk driver on FreeBSD Message-ID: <Pine.GSO.4.50L0.0405020024040.23508-100000@faith.cs.utah.edu>
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Hi, I am working on a Copy on Write disk driver on FreeBSD where I try to save the state of a filesystem (/dev/ad0s3) to another device (/dev/ad0s4) by making a virtual device that sits on top of these two (/dev/shd0). 1. So in the strategy routine, I get the block read/write calls to (/dev/shd0) . 2. For a write operation, I copy the previous contents of the block (number corresponding to /dev/ad0s3) on to a free block on /dev/ad0s4 3. To restore previous contents of disk, I read the allocated free block from /dev/ad0s4 and write it back to original block number /dev/ad0s3. The virtual device /dev/shd0 is mounted on /mnt So to test it out, my /dev/ad0s3 originally had a file "old1" of 13685 bytes containing repeating string pattern (OLDOLD) I then copied a file "new1" of 8211 bytes having the repeating pattern (NEWNEW) to overwrite the old one i.e. cp new1 /mnt/old1 A hexdump shows that a block of 8192 bytes containing "OLDOLD" was copied over to /dev/ad0s4 and its place being taken be "NEWNEW" in /dev/ad0s3. Also remaining bytes (beyond the 8192 bytes) still remain in /dev/ad0s3. So this shows that the copy on write was done correctly. And I correctly see 8211 bytes of "NEWNEW" in /mnt/old1 (ls -l /mnt/old1) I then send an IOCTL to my driver to restore to the previous state (expecting it to give me 13685 bytes of "OLDOLD" back in /mnt/old1) After unmounting and remounting, I see that the contents of /mnt/old1 have become OLDOLD, but there are only 8211 bytes instead of 13685. A hexdump of /dev/ad0s3 however, shows that there are indeed 13685 consecutive bytes of OLDOLD lying there. This has lead me to believe that the Inode of /mnt/old1 is not being refereshed (or it was never saved off to the /dev/ad0s4 in the first place). Do Inode read/writes go through the strategy routine in the first place? Any idea what could be going wrong? Thanks.
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