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Date:      Fri, 08 Dec 2023 13:53:37 +0000
From:      bugzilla-noreply@freebsd.org
To:        bugs@FreeBSD.org
Subject:   [Bug 275632] process being killed which has a big file mmap'ed and performs writes to it
Message-ID:  <bug-275632-227-TtuHz7nruW@https.bugs.freebsd.org/bugzilla/>
In-Reply-To: <bug-275632-227@https.bugs.freebsd.org/bugzilla/>
References:  <bug-275632-227@https.bugs.freebsd.org/bugzilla/>

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D275632

--- Comment #3 from Martin Birgmeier <d8zNeCFG@aon.at> ---
The issue is certainly reproducible - it happens every time I let the progr=
am
run.

Some background: "cawic" stands for "compare and write if changed". It is
basically a fancy way of copying stdin to a pre-existing file, but only tho=
se
portions which have changed are actually written. The pre-existing file is
actually a disk image; it is being updated from the physical disk every few
months (backup).

The 128 block size was chosen to match the ZFS default block size. Writing =
only
changed portions is intended to minimize snapshot sizes of the ZFS filesyst=
em
the file resides on.

The current workaround is to restart the program until it runs to completio=
n -
the parts which have already been written will only be read, then the next =
part
is partially rewritten and either fails there or finally succeeds.

-- Martin

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