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Forum Discussion
Scrubbing a single disk?
Hi
I use ReadyNAS 314 and run the latest firmware 6.4.1.
I have read through the manual and I just would like to understand something more of scrubbing.
I also googled around a bit and read for instanc the concise Wikipedia article on scdrubbing.
1) In the manual there is now distinction of scrubbing a single disk volume and a double disk raid 1 volume (or even higher raid configuration). In the raid1 situation I can imagine that one can correct an error on one disk by checking the other disk. However, a complication is of course to know which disk has the error and which is correcdt, but I guess by keeping track of when writes were done or similar then one can deduce which disk is correct. But I would appreciate a concise explanation of how this really works.
2) But for a single disk volume how can you scrub that disk? My guess is that here scrub means a much more limited check of the file system and the healthyness of the disk. Or is data redundantly stored on the single disk in a btrfs system and that only when the disk starts to get full (with the real data and not shadow copies for integrity checks) then the scrub get limited?
Curious!
Thanks
8 Replies
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A btrfs scrub will read all the data stored in the volume, and verify the btrfs checksums. If there is no raid redundancy, of course there can't be any repair of issues found. But it still confirms that (a) all files can be read and (b) the checksums are correct.
My btrfs volumes are all jbod, and I run scrubs on each volume every three months.
Thank you Stephen!
I also found two links that gives slightly expanded information:
The Wikipedia BTRFS article - and go the secdtion Checksum tree and scrubbing:
https://en.wikipedia.org/wiki/Btrfs#Checksum_tree_and_scrubbing
Also the Stack Overflow site has couple of texts worth reading for a layman.
http://superuser.com/questions/904331/how-does-btrfs-scrub-work-and-what-does-it-do/1008260
janpeter1 wrote:
Also the Stack Overflow site has couple of texts worth reading for a layman.
http://superuser.com/questions/904331/how-does-btrfs-scrub-work-and-what-does-it-do/1008260
I don't recommend this one. The commments on "burning in hard drives" and real-world disk reliability are questionable at best.
From a layman perspective, the key things are
(a) no single device or service (disk, NAS, PC, cloud, etc) is 100% reliable, so if you put your data on just one device you will eventually lose it.
The implication is that you need at least two copies (original and one backup) on different devices of anything you really care about. I recommend three copies (and I ignore RAID redundancy for this).
(b) It's important to identify disk problems early, especially with RAID arrays.
One implication here is that you should set up a volume maitenance schedule that includes regular disk tests, scrubs, balances, etc. And that you should set up email alerts on the NAS so that you are told about any issues that it sees. It is also important to regularly verify that your backups can still be read (I learned this one the hard way).
A second implication is that when the NAS does tell you about disk problems, you should address them right away. Delay adds to the risk.
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