Re: Using btrfs raid5/6

[Andrei Borzenkov <arvidjaar@gmail.com>]

05.12.2024 01:34, Qu Wenruo wrote:
> 
> 
> 在 2024/12/5 05:47, Andrei Borzenkov 写道:
>> 04.12.2024 07:40, Qu Wenruo wrote:
>>>
>>>
>>> 在 2024/12/4 14:04, Scoopta 写道:
>>>> I'm looking to deploy btfs raid5/6 and have read some of the previous
>>>> posts here about doing so "successfully." I want to make sure I
>>>> understand the limitations correctly. I'm looking to replace an md+ext4
>>>> setup. The data on these drives is replaceable but obviously ideally I
>>>> don't want to have to replace it.
>>>
>>> 0) Use kernel newer than 6.5 at least.
>>>
>>> That version introduced a more comprehensive check for any RAID56 RMW,
>>> so that it will always verify the checksum and rebuild when necessary.
>>>
>>> This should mostly solve the write hole problem, and we even have some
>>> test cases in the fstests already verifying the behavior.
>>>
>>
>> Write hole happens when data can *NOT* be rebuilt because data is
>> inconsistent between different strips of the same stripe. How btrfs
>> solves this problem?
> 
> An example please.

You start with stripe

A1,B1,C1,D1,P1

You overwrite A1 with A2

Before you can write P2, system crashes

After reboot D goes missing, so you now have

A2,B1,C1,miss,P1

You cannot reconstruct "miss" because P1 does not match A2. You can 
detect that it is corrupted using checksum, but not infer the correct data.

MD solves it by either computing the extra parity or by buffering full 
stripe before writing it out. In both cases it is something out of band.

> 
>>
>> It probably can protect against data corruption (by verifying checksum),
>> but how can it recover the correct content?
>>
>