btrfs recovery: What do the commands actually do?

btrfs recovery: What do the commands actually do?

[Martin]

There's ad-hoc comment for various commands to recover from filesystem
errors.

But what do they actually do and when should what command be used?

(The wiki gives scant indication other than to 'blindly' try things...)


There's:

mount "-o recovery,noatime"

btrfsck:
    --repair                    try to repair the filesystem
    --init-csum-tree            create a new CRC tree
    --init-extent-tree          create a new extent tree

And there is "scrub"...


What do they do exactly and what are the indicators to try using them?

Or when should you 'give up' on a filsystem and just retrieve whatever
data can be read and start again?


All that lot sounds good for a wiki page ;-)

Thanks,
Martin

Re: btrfs recovery: What do the commands actually do?

[Duncan]

Martin posted on Fri, 04 Oct 2013 16:47:19 +0100 as "condensed":

> There's ad-hoc comment for various commands to recover from filesystem
> errors.
> 
> But what do they actually do and when should what command be used?
> What do they do exactly and what are the indicators to try using them?
> Or when should you 'give up' on a filsystem and just retrieve whatever
> data can be read and start again?
> 
> All that lot sounds good for a wiki page ;-)

I recognize your name so you're a regular poster and may well have seen 
this recover steps/order post from Hugo Mills, but you didn't mention it, 
so...

http://permalink.gmane.org/gmane.comp.file-systems.btrfs/27999

As you suggest, that should really go in the wiki (maybe it's there 
already since that post, I haven't actually checked recently, but your 
post reads as if you looked and couldn't find a recovery list of this 
nature), but I've not gotten around to creating an account for myself 
there yet and committing it, and if no one else has either...

But I do have it bookmarked for posting here, and for the day I do create 
myself that wiki account, if no one else has gotten to it by then...

And while that answers what and what order, it doesn't cover what the 
commands actually do or why you'd /use/ that order, and that'd be very 
good to add as well.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

Re: btrfs recovery: What do the commands actually do?

[Martin]

On 04/10/13 19:32, Duncan wrote:
> Martin posted on Fri, 04 Oct 2013 16:47:19 +0100 as "condensed":
> 
>> There's ad-hoc comment for various commands to recover from filesystem
>> errors.
>>
>> But what do they actually do and when should what command be used?
>> What do they do exactly and what are the indicators to try using them?
>> Or when should you 'give up' on a filsystem and just retrieve whatever
>> data can be read and start again?
>>
>> All that lot sounds good for a wiki page ;-)
> 
> I recognize your name so you're a regular poster and may well have seen

Hail fellow Gentoo-er ;-)


This is a prod from the thread:

http://article.gmane.org/gmane.comp.file-systems.btrfs/28775


> this recover steps/order post from Hugo Mills, but you didn't mention it, 
> so...
> 
> http://permalink.gmane.org/gmane.comp.file-systems.btrfs/27999
> 
> As you suggest, that should really go in the wiki (maybe it's there 
> already since that post, I haven't actually checked recently, but your 
> post reads as if you looked and couldn't find a recovery list of this 
> nature), but I've not gotten around to creating an account for myself 
> there yet and committing it, and if no one else has either...
> 
> But I do have it bookmarked for posting here, and for the day I do create 
> myself that wiki account, if no one else has gotten to it by then...
> 
> And while that answers what and what order, it doesn't cover what the 
> commands actually do or why you'd /use/ that order, and that'd be very 
> good to add as well.

Yep.

I'm using this as a bit of a test case as to how best to recover from
whatever inevitable hiccups.

All the more important to gain a good understanding before doing similar
things to 16TB arrays...


Comment/advice welcomed (please).

Thanks,
Martin