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

From: Andreas Schweigstill <andreas_at_schweigstill.de>
Date: Mon, 16 Feb 2009 13:22:51 +0100


Jan Hendrik schrieb:
> Funny thing though: it only happens when the files already exist
> on the Linkstation, any "new" file invariably gets the timestamp
> it has on the local machine.

There are several ways to write to an existing file. Many editors
use a scheme where they first rename the old file to a temporary
file, then they write their buffer contents to a new file which gets
the old filename, and finally removes the old file with the temporary
filename. This way it is assured that the file can be completely
erased because of a program crash or network problem. In such cases
the user would at least find the old file. When copying the original
file attributes after successfully writing the new file probably
also the old timestamp gets copied. This can either be related to
some caching issues or just a simple programming error.

On some RAID systems there is also a "feature" which is called
WOW (write on write) problem. When a block gets twice in a short
time to a mirrored disk it can happen that write operation 1 gets
executed on mirropr 1 first and on mirror 2 afterwards. Write op. 2
gets executed the other way. Due to this race condition there
can be inconsistent states on the mirrors. Similar race conditions
on mirrored disks can occur when write and read operations are

Nowadays these WOW problems "should" be non-existing but I am not
dure since when Linux soft-RAID handles them correctly.

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 2009-02-16 13:25:17 CET

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