Postmortem of last week's fileserver failure | Hacker News Reader

Postmortem of last week's fileserver failure

github.com

43 pointsmnemonik9 comments

Server-side data loss always sucks, but this would be much harder to recover from with a non-distributed VCS. Another win for the D.

I absolutely love postmortems. I think it's because I love seeing the complex ways our dear friend Murphy works. Also, it's really nice to be able to see problems other people run into, so you can avoid them or plan for them yourself.

I find candid postmortems indicative of a dependable service. Good work Github.

THIS is how you handle failure. You learn from it, and let others learn as well.

I love how up front those guys are.

This is a classic example of how the naieve plan of 'Get two servers and mirror them' can lead to downtime regardless of the best intentions of sysadmin.

Maybe the best comment about this issue.

The problem with the GitHub RAID approach + daily snapshot is that you end up with only the equivalent of a daily backup and you do not have data integrity insurance.

Each of their file storage pair is running in RAID + RAID over the network (DRBD). RAID is not a backup because you can get data corruption and you just replicate the corruption.

Imagine that they got part of their A server having issues, which are with current FS+RAID simply ignored. You can run months without integrity check even with Git because if you add new objects and do not need to repack, no integrity check is performed on the old objects. So your corruption spread from A to B (RAID) to backup (daily snapshot). Then you need at home to checkout because you lost your HD, you are doomed. Full data loss without a single warning. If you are a single developer on your own private project, this scenario is easy to achieve.

I really really like the way Fastmail is doing backup. They are replaying the IMAP operations with checksum. With Git you can also replay with checksum too. In fact, it is built in thanks to the DSCM nature of Git.

For the backup of my customer git repositories, this is what I am doing, in the post-update hook, I fire a git sync with git on another server, it is doing everything needed while being CPU+bandwidth efficient. Thank you git.

The easy way to do it: http://kerneltrap.org/mailarchive/git/2007/10/18/346839

> Prior to the failure, we had been seeing some anomalous behaviour from the A server in this pair. The machine had been taken offline and memory tests performed, but no issue was found.

Presumably the anomalous behavior was disk-related and yet they only test memory.

Why not check SMART status? Or if the RAID driver doesn't support SMART passthrough; move the drives to the SATA controller and boot off of a flash drive.

Also, there are tools to generate lots of i/o and test for failures, i.e.:

http://www.peereboom.us/iogen/

Damn, that is sluppy. GitHub is, essentially a glorified file storage place and they don't have a wy to ensure th at corruption doesn't happen?

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