So my Fedora Linux machine crashed during an update, and now refuses to start properly. I'm using SystemRescueCD to try to recover the contents of the hard drive. Following the steps in this post, I have done the following, shown together with the respective output for each command.First I list the partitions:root@sysresccd /root % fdisk -lDisk /dev/loop0: 338.5 MiB, 354885632 bytes, 693136 sectorsUnits: sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytesDisk /dev/sda: 477 GiB, 512110190592 bytes, 1000215216 sectorsUnits: sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytesDisklabel type: dosDisk identifier: 0x283f70c2Device     Boot   Start        End   Sectors   Size Id Type/dev/sda1  *       2048    1026047   1024000   500M 83 Linux/dev/sda2       1026048 1000214527 999188480 476.5G 8e Linux LVMDisk /dev/mapper/fedora-swap: 7.6 GiB, 8187281408 bytes, 15990784 sectorsUnits: sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytesDisk /dev/sdb: 7.5 GiB, 8076132352 bytes, 15773696 sectorsUnits: sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytesDisklabel type: dosDisk identifier: 0x29ca9ce2Device     Boot Start     End Sectors  Size Id Type/dev/sdb1        2048 7641087 7639040  3.7G  b W95 FAT32The harddrive I want to access is in sda2, so I try to mount it.root@sysresccd /root % mkdir /mnt/oldroot@sysresccd /root % mount /dev/sda2 /mnt/old mount: unknown filesystem type 'LVM2_member'So it is unable to mount the hard drive because it does not recognise the filesystem.With the lvm2 tools, I do a disk scanroot@sysresccd /root % lvmdiskscan/dev/loop0              [     338.45 MiB] /dev/mapper/fedora-swap [       7.62 GiB] /dev/sda1               [     500.00 MiB] /dev/sda2               [     476.45 GiB] LVM physical volume/dev/sdb1               [       3.64 GiB] 1 disk3 partitions0 LVM physical volume whole disks1 LVM physical volumeWith lvdisplay I get the logical volume (LV) name  and the volume group (VG) nameroot@sysresccd /root % lvdisplayWARNING: Unrecognised segment type thin-poolWARNING: Unrecognised segment type thin--- Logical volume ---LV Path                /dev/fedora/pool00LV Name                pool00VG Name                fedoraLV UUID                Ye2FvY-Sx80-znoh-aYdi-Q5wM-e0W3-UPaQtALV Write Access        read/writeLV Creation host, time localhost, 2016-01-04 15:59:45 +0000LV Status              NOT availableLV Size                452.82 GiBCurrent LE             115922Segments               1Allocation             inheritRead ahead sectors     auto--- Logical volume ---LV Path                /dev/fedora/rootLV Name                rootVG Name                fedoraLV UUID                DLcLQA-VcRn-u7fQ-sWaL-v9cY-M5EW-F3ZFuNLV Write Access        read/writeLV Creation host, time localhost, 2016-01-04 15:59:46 +0000LV Status              NOT availableLV Size                50.00 GiBCurrent LE             12800Segments               1Allocation             inheritRead ahead sectors     auto--- Logical volume ---LV Path                /dev/fedora/homeLV Name                homeVG Name                fedoraLV UUID                aTrVab-urfB-u0xU-zoit-PK8H-l5Sf-2MfaXVLV Write Access        read/writeLV Creation host, time localhost, 2016-01-04 15:59:48 +0000LV Status              NOT availableLV Size                402.82 GiBCurrent LE             103122Segments               1Allocation             inheritRead ahead sectors     auto--- Logical volume ---LV Path                /dev/fedora/swapLV Name                swapVG Name                fedoraLV UUID                MuFrai-TMdG-uiap-y7fh-5lhU-dYlL-cjjBAZLV Write Access        read/writeLV Creation host, time localhost, 2016-01-04 15:59:51 +0000LV Status              available# open                 0LV Size                7.62 GiBCurrent LE             1952Segments               1Allocation             inheritRead ahead sectors     auto- currently set to     256Block device           253:0The vgdisplay command also gives similar informationroot@sysresccd /root % vgdisplayWARNING: Unrecognised segment type thin-poolWARNING: Unrecognised segment type thin--- Volume group ---VG Name               fedoraSystem ID             Format                lvm2Metadata Areas        1Metadata Sequence No  9VG Access             read/writeVG Status             resizableMAX LV                0Cur LV                4Open LV               0Max PV                0Cur PV                1Act PV                1VG Size               476.45 GiBPE Size               4.00 MiBTotal PE              121971Alloc PE / Size       117932 / 460.67 GiBFree  PE / Size       4039 / 15.78 GiBVG UUID               WduLzz-NwqH-DXYy-8fQy-ojos-SDi4-EmmHs5Now I tried a new mount, using the LV name path:root@sysresccd /root % mount /dev/fedora/home /mnt/old mount: special device /dev/fedora/home does not existIt still refuses to mount.lvscan shows the status of the logical volumeroot@sysresccd /root % lvscanWARNING: Unrecognised segment type thin-poolWARNING: Unrecognised segment type thininactive          '/dev/fedora/pool00' [452.82 GiB] inheritinactive          '/dev/fedora/root' [50.00 GiB] inheritinactive          '/dev/fedora/home' [402.82 GiB] inheritACTIVE            '/dev/fedora/swap' [7.62 GiB] inheritAs you can see, it is still inactive and not mounted. Also, there are two warnings about unrecognised segment types.So even though I continue with the given instructions and add the device mapping module dm-mod to the kernel:root@sysresccd /root % modprobe dm-modThen I change the attributes of the volume group:root@sysresccd /root % vgchange -ayWARNING: Unrecognised segment type thin-poolWARNING: Unrecognised segment type thinRefusing activation of LV pool00 containing an unrecognised segment.Refusing activation of LV root containing an unrecognised segment.Refusing activation of LV home containing an unrecognised segment.1 logical volume(s) in volume group fedora now activeBut the attributes do not change due to the unrecognised segments, and the logical volumes stay inactive.root@sysresccd /root % lvscanWARNING: Unrecognised segment type thin-poolWARNING: Unrecognised segment type thininactive          '/dev/fedora/pool00' [452.82 GiB] inheritinactive          '/dev/fedora/root' [50.00 GiB] inheritinactive          '/dev/fedora/home' [402.82 GiB] inheritACTIVE            '/dev/fedora/swap' [7.62 GiB] inheritI don't know what thin and thin-pool mean in this context, but it seems quite clear that they are blocking the access to the old partitions. So if there is anyone who can spot the problem, please tell how to solve it.
mount: Unrecognised segment type thin
filesystems;mount;partition;lvm;system recovery
This appears to be a bug in older versions of LVM.  A bug that could be corrected by compiling from source with a different set of flags to add support for thin devices.  I can not speak for the SystemRescueCD you mentioned, because I have never used it, but it may be using an older version of LVM, for whatever reason, which may have this very bug.Since you mentioned you are running Fedora, have you tried getting an official Fedora ISO image to boot from?  Grab the server version here: https://getfedora.org/en/server/download/I suggest the server version rather than desktop version because of the options for troubleshooting available.Simply boot from the ISO image (CD/DVD or USB thumb drive) and start rescue mode.  Version 23 of the Fedora server ISO I tested seemed to have no issues reading any LVM volumes on a test machine I purposely broke the file systems on.  Of course, your mileage may vary. :\I have also had success with thin-provisioning-tools found here: https://github.com/jthornber/thin-provisioning-toolsAfter booting from the Fedora ISO, you may need to do a bit of leg work to get your machine to boot up far enough to install the tools.  Perhaps not mounting the damaged mount points at boot, if possible that is.