10
Aug
2015

BTRFS USB Array: Errors Found!

So I ran a scrub on my USB drives in a BTRFS RAID-1 mode (more details here), and I got the following output:

Screenshot_2015-08-09_23-16-23

1 error was detected! Fortunately, this error was corrected for me though! But, which drive was this on? You can find this out too on the main page of the Webmin module:

Screenshot_2015-08-09_23-18-06

The module also gave me this information:

[ 1431.445495] BTRFS: checksum error at logical 374177792 on dev /dev/sdb, sector 730816: metadata leaf (level 0) in tree 5

Hmm, drive sdb was corrupted. This was the smaller of the two sticks (the top Integral one in this image):

DSCF0212_small

It’s a good job I decided to go for BTRFS: these drives contain very important data and I am glad that I have it there to protect me against corruption! I’m not relying solely on these USB drives though: the data is replicated on my RAID-5 array and also on other machines that I own, so I am pretty well protected! If I see more errors in the future, I will be replacing these drives with larger ones (since I am running out of space).

So, in conclusion: don’t use mdadm for RAID-1 with critical data. There is no protection from data corruption or bit-rot. For that, you need a modern filesystem that stores checksums alongside the data, and constantly checks the data for errors as it is read by the OS. I do not know if I would have lost any data with mdadm in this situation, but its better to be safe than sorry!

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