Hi Jon, thanks a bunch!! that explains it now. -Aravind. --- Jon Burgess <jburgess777@xxxxxxxxxxxxxx> wrote: > On Thu, 2007-04-19 at 23:19 -0700, Aravindan > Raghuveer wrote: > > Hello! > > > > I had recently posted a question on this list > about > > getting LBAs for a file on ext3. Andreas Dilger > had > > provided a very handy solution to solve the above > > problem [Message attached along]. > > > > My goal is to be able to read files on a ext3 > > filesystem through the scsi generic (sg) driver. > > I first use debugfs to extract the block addresses > for > > every file. I then multiply the block address by 8 > > [4K/ 512] to obtain the LBAs. I then use a progam > > sg_dd in the sg toolkit to read the data stored in > the > > LBA obtained above. > > > > However, the data I read through the sg driver > does > > not match the data in the file. Through some > tinkering > > of my code, I found that when I offset the > original > > LBA [obtained as block_address*8] by 63 sectors > > [63*512 bytes], the data perfectly matches. That > is > > LBA' = LBA + 63 corresponds to the actual address > used > > for the file. I have tested this well when the > file is > > not fragmented. Initial testing on fragmented > files > > also shows similar behavior. > > > > Are you accessing the raw disk or the partition > containing the > filesystem? 63 sounds suspiciously like the typical > sector offset of the > first partition on the disk. The filesystem block > numbers are relative > to the containing partition. > > > # sfdisk -l -uS /dev/sda > > Disk /dev/sda: 24321 cylinders, 255 heads, 63 > sectors/track > Units = sectors of 512 bytes, counting from 0 > > Device Boot Start End #sectors Id > System > /dev/sda1 * 63 2104514 2104452 fd > Linux raid autodetect > /dev/sda2 2104515 390716864 388612350 fd > Linux raid autodetect > /dev/sda3 0 - 0 0 > Empty > /dev/sda4 0 - 0 0 > Empty > > > > I googled around but could not find a convincing > > answer as to where this offset crops up from. Has > > anyone in the group done a similar experiment? I > would > > really appreciate any pointers on this issue. > > > > thanks again, > > > > regards, > > > > -Aravind. > > > > > > --- Andreas Dilger <adilger@xxxxxxxxxxxxx> wrote: > > > > > On Apr 16, 2007 11:17 -0700, Aravindan > Raghuveer > > > wrote: > > > > I need to write a user space tool that can > dump > > > > logical block addresses used by every file in > a > > > ext3 > > > > file system. For example, if file foo uses > LBAs > > > 2,3 > > > > and file bar uses LBAs 100,102,156, then the > > > ouptut > > > > should read: > > > > > > > > FILENAME LBAs > > > > foo 2, 3 > > > > bar 100,102,156 > > > > > > > > Is there a tool that exists that can do this? > If > > > not, > > > > what would be a good strategy to write this > tool. > > > I am > > > > learning filesystem programming and would > really > > > > appreciate any pointers. > > > > > > "debugfs -c -R 'stat /path/to/file' /dev/XXX" > > > reports, among other things > > > the blocks used by that file. > > > > > > Cheers, Andreas > > > -- > > > Andreas Dilger > > > Principal Software Engineer > > > Cluster File Systems, Inc. > > > > > > > > > > > > __________________________________________________ > > Do You Yahoo!? > > Tired of spam? Yahoo! Mail has the best spam > protection around > > http://mail.yahoo.com > > > > _______________________________________________ > > Ext3-users mailing list > > Ext3-users@xxxxxxxxxx > > https://www.redhat.com/mailman/listinfo/ext3-users > > __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com _______________________________________________ Ext3-users mailing list Ext3-users@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/ext3-users
