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From: waxhead <>
To: DanglingPointer <>,
Subject: Re: RAID56 Warning on "multiple serious data-loss bugs"
Date: Sat, 26 Jan 2019 13:07:41 +0100
Message-ID: <> (raw)
In-Reply-To: <>

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: 
> 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.

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

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