Re: csum failed root raveled during balance

[ein <ein.net@gmail.com>]

On 05/23/2018 11:09 AM, Duncan wrote:
> ein posted on Wed, 23 May 2018 10:03:52 +0200 as excerpted:
> 
>>> IMHO the best course of action would be to disable checksumming for you
>>> vm files.
>>
>> Do you mean '-o nodatasum' mount flag? Is it possible to disable
>> checksumming for singe file by setting some magical chattr? Google
>> thinks it's not possible to disable csums for a single file.
> 
> You can use nocow (-C), but of course that has other restrictions (like 
> setting it on the files when they're zero-length, easiest done for 
> existing data by setting it on the containing dir and copying files (no 
> reflink) in) as well as the nocow effects, and nocow becomes cow1 after a 
> snapshot (which locks the existing copy in place so changes written to a 
> block /must/ be written elsewhere, thus the cow1, aka cow the first time 
> written after the snapshot but retain the nocow for repeated writes 
> between snapshots).
> 
> But if you're disabling checksumming anyway, nocow's likely the way to go.

Disabling checksumming only may be a way to go - we live without it
every day. But nocow @ VM files defeats whole purpose of using BTRFS for
me, even with huge performance penalty - backup reasons - I mean few
snapshots (20-30), send & receive.


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