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Backup and restore lvm data with dd

Sun, 03/13/2011 - 01:18 — sandip

Recently I've had to backup/restore data from a failing drive with LVM over Raid.

Luckily I had access to the backup of the current metadata configuration located in "/etc/lvm/backup/".

Below is what the volumegroup looked like:

vg0 {
        id = "xvni1W-24Xu-dVoR-PlXh-gQvQ-62fL-QX64O3"
        seqno = 9
        status = ["RESIZEABLE", "READ", "WRITE"]
        flags = []
        extent_size = 65536             # 32 Megabytes
        max_lv = 0
        max_pv = 0

        physical_volumes {

                pv0 {
                        id = "9gbyhX-Owvj-u4Q4-wR1E-IEf2-gyUA-CJBCJK"
                        device = "/dev/md3"     # Hint only

                        status = ["ALLOCATABLE"]
                        flags = []
                        dev_size = 1928892288   # 919.768 Gigabytes
                        pe_start = 384
                        pe_count = 29432        # 919.75 Gigabytes
                }
        }

        logical_volumes {

                lv0_sites {
                        id = "Sg1fYr-NTzr-8AA2-v29K-tcz5-rUMj-uRoXY1"
                        status = ["READ", "WRITE", "VISIBLE"]
                        flags = []
                        segment_count = 1

                        segment1 {
                                start_extent = 0
                                extent_count = 1280     # 40 Gigabytes

                                type = "striped"
                                stripe_count = 1     ;   # linear

                                stripes = [
                                        "pv0", 0
                                ]
                        }
                }

                lv0_m {
                        id = "scNeN4-4bmg-Y6kq-zKuO-n8B8-s8mw-FTUYqk"
                        status = ["READ", "WRITE", "VISIBLE"]
                        flags = []
                        segment_count = 1

                        segment1 {
                                start_extent = 0
                                extent_count = 12800    # 400 Gigabytes

                                type = "striped"
                                stripe_count = 1     ;   # linear

                                stripes = [
                                        "pv0", 1280
                                ]
                        }
                }
        }
}

Now to extract data with dd, use the below formula (this will only work for linear stripe):

skip=$[extent_size*stripes+pe_start] count=$[extent_size*(extent_count-1)]

So to get the lv0_m data off of the volume:

dd if=/dev/sdb4 of=/opt/bak/lv0_m.iso bs=512 skip=$[65536*1280+384] count=$[65536*(12800-1)] conv=sync,noerror

Once the iso is created, it can then be loop mounted via:

mount -o loop -t ext3 /opt/bak/lv0_m.iso /mnt/lv0_m

You should then be able to see all the files in the mount point which can then be used for data restoration.

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