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From: Goffredo Baroncelli <kreijack@inwind.it>
To: Qu Wenruo <quwenruo@cn.fujitsu.com>
Cc: Chris Murphy <lists@colorremedies.com>,
	waxhead <waxhead@dirtcellar.net>,
	Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: Exactly what is wrong with RAID5/6
Date: Thu, 22 Jun 2017 07:15:22 +0200	[thread overview]
Message-ID: <896c9f64-68ca-7112-2387-24028f4628c3@inwind.it> (raw)
In-Reply-To: <1d88f12a-bd2a-9fff-cb53-da847f4526c7@cn.fujitsu.com>

On 2017-06-22 04:12, Qu Wenruo wrote:
> 
> And in that case even device of data stripe 2 is missing, btrfs don't really need to use parity to rebuild it, as btrfs knows there is no extent in that stripe, and data csum matches for data stripe 1.

You are assuming that there is no data in disk2. This is likely, due to COW nature of BTRFS. But it is not always true. 

Anyway, the same problem happens if you are writing data on disk2 . If 
a) data (disk2) is written 
b) parity is not updated (due to a power failure)

until that you don't lose anything, but if

c) disk1 disappear 

you are not in position to recompute valid data in disk1 using only data2 and parity


> No need to use parity at all.
> 
> So that's why I think the hole write is not an urgent case to handle right now.
> 
> Thanks,
> Qu


-- 
gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it>
Key fingerprint BBF5 1610 0B64 DAC6 5F7D  17B2 0EDA 9B37 8B82 E0B5

  parent reply	other threads:[~2017-06-22  5:15 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-06-20 22:57 Exactly what is wrong with RAID5/6 waxhead
2017-06-20 23:25 ` Hugo Mills
2017-06-21  3:48   ` Chris Murphy
2017-06-21  6:51     ` Marat Khalili
2017-06-21  7:31       ` Peter Grandi
2017-06-21 17:13       ` Andrei Borzenkov
2017-06-21 18:43       ` Chris Murphy
2017-06-21  8:45 ` Qu Wenruo
2017-06-21 12:43   ` Christoph Anton Mitterer
2017-06-21 13:41     ` Austin S. Hemmelgarn
2017-06-21 17:20       ` Andrei Borzenkov
2017-06-21 17:30         ` Austin S. Hemmelgarn
2017-06-21 17:03   ` Goffredo Baroncelli
2017-06-22  2:05     ` Qu Wenruo
2017-06-21 18:24   ` Chris Murphy
2017-06-21 20:12     ` Goffredo Baroncelli
2017-06-21 23:19       ` Chris Murphy
2017-06-22  2:12     ` Qu Wenruo
2017-06-22  2:43       ` Chris Murphy
2017-06-22  3:55         ` Qu Wenruo
2017-06-22  5:15       ` Goffredo Baroncelli [this message]
2017-06-23 17:25 ` Michał Sokołowski
2017-06-23 18:45   ` Austin S. Hemmelgarn

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