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[SOLVED] Recover bcache from previous system?
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statikregimen
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Joined: 16 Jul 2011
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Location: USA/Michigan

PostPosted: Fri Aug 25, 2017 2:37 pm    Post subject: [SOLVED] Recover bcache from previous system? Reply with quote

EDIT: A little more google-fu has lead me here https://stackoverflow.com/questions/22820492/how-to-revert-bcache-device-to-regulare-filesystem#23295521

It looks very promising. I am reading it now and will try the suggestion(s), reporting back my findings.

[SOLVED]
Code:
# losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev
# mount /dev/loop0 /mnt/somewhere
# cd /mntsomewhere
# ls
<your data should now be listed here>


It is up to you what to do with it next. I'm going to back it up and get rid of those gross NTFS partitions, then make a new bcache for my /home directory :D

p.s. I still can't get over how 99% of the time, right after I give up and post in forums, I find the solution on my own....

Original post:
Good day,

I recently decided to move from Ubuntu to Gentoo

The machine has two SSDs, split into two mdadm RAID0 arrays.

The first array is for the root partition. On Ubuntu, I was using the 2nd array as cache for a spinning disk.

So my question is: Is it possible to bring the cache+backing drive back online with data in-tact? I've google-fu'ed the crap out of it, but the best I can find is how to cache a device that contains existing data. It's not the end of the world if I can't, which is why I didn't bother backing anything up first. But in retrospect, it would be nice to have that data back...would save me redownloading some big files, and forgot about a couple save games that were on there.

I've tried simply mounting the backing device:
Code:
# mount /dev/sda2 /media/tmp/
mount: unknown filesystem type 'bcache'


Recreating the device:
Code:
# make-bcache -B /dev/sda2
Already a bcache device on /dev/sda2, overwrite with --wipe-bcache


It apparently doesn't get automatically detected:
Code:
# ls /dev/bcache*
ls: cannot access '/dev/bcache*': No such file or directory


I have set in my kernel config:
Code:

<*>   Block device as cache
[ ]     Bcache debugging
[ ]     Debug closures


Code:
# lsblk
NAME          MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sr0            11:0    1  1024M  0 rom   
nvme1n1       259:4    0 119.2G  0 disk 
├─nvme1n1p3   259:6    0  59.4G  0 part 
│ └─md127       9:127  0 118.7G  0 raid0
└─nvme1n1p2   259:5    0  59.4G  0 part 
  └─md126       9:126  0 118.7G  0 raid0
    ├─md126p3 259:9    0   6.9G  0 md    [SWAP]
    ├─md126p1 259:7    0  18.6G  0 md    /
    └─md126p2 259:8    0  93.1G  0 md   
sda             8:0    0 931.5G  0 disk 
├─sda2          8:2    0 931.4G  0 part 
└─sda1          8:1    0   100M  0 part 
nvme0n1       259:0    0 119.2G  0 disk 
├─nvme0n1p3   259:3    0  59.4G  0 part 
│ └─md127       9:127  0 118.7G  0 raid0
├─nvme0n1p1   259:1    0   512M  0 part  /boot
└─nvme0n1p2   259:2    0  59.4G  0 part 
  └─md126       9:126  0 118.7G  0 raid0
    ├─md126p3 259:9    0   6.9G  0 md    [SWAP]
    ├─md126p1 259:7    0  18.6G  0 md    /
    └─md126p2 259:8    0  93.1G  0 md


Code:
# fdisk -l /dev/sda
Device     Boot  Start        End    Sectors   Size Id Type
/dev/sda1  *      2048     206847     204800   100M  7 HPFS/NTFS/exFAT
/dev/sda2       206848 1953523119 1953316272 931.4G  7 HPFS/NTFS/exFAT



I'm happy to provide any further details if needed.

Thanks in advance!
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