Linux-BTRFS Archive on lore.kernel.org
 help / color / Atom feed
From: waxhead <waxhead@dirtcellar.net>
To: DanglingPointer <danglingpointerexception@gmail.com>,
	linux-btrfs@vger.kernel.org
Subject: Re: RAID56 Warning on "multiple serious data-loss bugs"
Date: Sat, 26 Jan 2019 13:07:41 +0100
Message-ID: <f7f5d96f-2650-9162-02ef-c9200364c86a@dirtcellar.net> (raw)
In-Reply-To: <5d7f63b2-d340-7c3a-679b-26e97ac258a6@gmail.com>

DanglingPointer wrote:
> 
> Hi All,
> 
> For clarity for the masses, what are the "multiple serious data-loss 
> bugs" as mentioned in the btrfs wiki?
> The bullet points on this page: 
> https://btrfs.wiki.kernel.org/index.php/RAID56
> don't enumerate the bugs.  Not even in a high level.  If anything what 
> can be closest to a bug or issue or "resilience use-case missing" would 
> be the first point on that page.
> 
> "Parity may be inconsistent after a crash (the "write hole"). The 
> problem born when after "an unclean shutdown" a disk failure happens. 
> But these are *two* distinct failures. These together break the BTRFS 
> raid5 redundancy. If you run a scrub process after "an unclean shutdown" 
> (with no disk failure in between) those data which match their checksum 
> can still be read out while the mismatched data are lost forever."
> 
> So in a nutshell; "What are the multiple serious data-loss bugs?" If 
> there aren't any, perhaps updating the wiki should be considered for 
> something less the "dramatic" .
> 
I would just like to add that according to the status page the only 
missing piece from a implementation point of view is the write hole.
https://btrfs.wiki.kernel.org/index.php/Status#RAID56

What effect exactly the write hole might have on *data* is not pointed 
out in detail, but I would imagine that for some it might be desirable 
to run a btrfs filesystem with metadata in "RAID" 1/10 mode and data in 
"RAID" 5/6.

As far as I can understand this would leave you in a position where your 
filesystem structures are relatively safe as "RAID" 1/10 mode is 
considered stable. e.g. you should not loose or corrupt your filesystem 
in the event of a crash / brownout. On the other hand you might loose or 
corrupt a file being written which may or may not be acceptable for 
some. In any case a scrub should fix any inconsistencies.

My point being that such a configuration might be (just?) as safe as for 
exampel mdraid 5/6 and in some cases perhaps even more thanks to 
checksumming and the self-heal features of btrfs.

Unless I am totally off I think it would be wise to add the metadata 
"RAID" 1/10 and data "RAID" 5/6 method to the wiki as a possible "no 
worse than any other XYZ solution" if you need storage and don't have 
that much metadata in your filesystem.

  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-26 11:45 DanglingPointer
2019-01-26 12:07 ` waxhead [this message]
2019-01-26 14:05   ` Remi Gauvin
2019-01-28  0:52 ` Qu Wenruo
2019-01-28 15:23   ` Supercilious Dude
2019-01-28 16:24     ` Adam Borowski
2019-01-28 22:07   ` DanglingPointer
2019-01-28 22:52     ` Remi Gauvin
2019-01-29 19:02       ` Chris Murphy
2019-01-29 19:47         ` Goffredo Baroncelli
2019-01-30  1:41           ` DanglingPointer
2019-02-01 18:45         ` Remi Gauvin
2019-01-29  1:46     ` Qu Wenruo

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=f7f5d96f-2650-9162-02ef-c9200364c86a@dirtcellar.net \
    --to=waxhead@dirtcellar.net \
    --cc=danglingpointerexception@gmail.com \
    --cc=linux-btrfs@vger.kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-BTRFS Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-btrfs/0 linux-btrfs/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-btrfs linux-btrfs/ https://lore.kernel.org/linux-btrfs \
		linux-btrfs@vger.kernel.org linux-btrfs@archiver.kernel.org
	public-inbox-index linux-btrfs


Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-btrfs


AGPL code for this site: git clone https://public-inbox.org/ public-inbox