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Re: 2 users working on same file caused a change to be reverted rather than merged

From: Toby Thain <toby_at_telegraphics.com.au>
Date: Wed, 18 Feb 2009 09:12:20 -0500

On 17-Feb-09, at 11:21 PM, Andreas Schweigstill wrote:

> Hello!
>
> Toby Thain schrieb:
>> This is a flaw of *all* RAID-1 systems, software or hardware
>> (excepting the few in the high end that implement checksums).
>
> No, it is only related to some OS which do some performance
> optimizations. On Solaris one can set md_mirror:md_mirror_wow_flg=0x20
> which disables performance optimization.
>
>> It also risks integrity if the system suffers a sudden interruption
>> during writing (e.g. powerfail, panic, etc).
>
> That is another problem which can't be completely solved. But this
> has nearly nothing to do with normal operation.
>
>>> Nowadays these WOW problems "should" be non-existing but I am not
>>> dure since when Linux soft-RAID handles them correctly.
>>
>> There is no solution; the flaw is inherent in the design. *Non-
>> checksummed* RAID-1 cannot determine which side of the mirror holds
>> valid data.
>
> They just can't determine the valid mirror side in the case of an
> interruption or for computing clusters which access the same disks.
>
> And for normal use it is not important that both sides of the mirror
> contain the same data also on short timescales. It is sufficient that
> read operations on the same sector/block return consistent data.

There are several ways the mirror sides can get out of sync
unbeknownst to the storage layer. Sudden interruption is only the
most common. RAID-1 users should be aware that in general there is no
recourse when it does.

--Toby

>
>> Sun's ZFS solves this problem entirely.
>
> ZFS solves many problems, but the WOW problem was/is mainly present on
> Solaris systems using the normal Sun Volume Manager with UFS.
>
> Regards
> Andreas Schweigstill
>
> --
> Dipl.-Phys. Andreas Schweigstill
> Schweigstill IT | Embedded Systems
> Schauenburgerstra├če 116, D-24118 Kiel, Germany
> Phone: (+49) 431 53035-435, Fax: (+49) 431 53035-436
> Mobile: (+49) 171 6921973, Web: http://www.schweigstill.de/
>
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Received on 2009-02-18 15:13:16 CET

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