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Date:      Mon, 14 Oct 2013 13:09:40 -0400
From:      Brad Mettee <bmettee@pchotshots.com>
To:        CeDeROM <cederom@tlen.pl>, Adam Vande More <amvandemore@gmail.com>
Cc:        David Demelier <demelier.david@gmail.com>, FreeBSD Questions <freebsd-questions@freebsd.org>
Subject:   Re: SU+J Lost files after a power failure
Message-ID:  <525C2554.7080203@pchotshots.com>
In-Reply-To: <CAFYkXjn-1wTJcQ4a_fyXCvwh9ukt3+djM2qsMeaH1HhVJNvhiA@mail.gmail.com>
References:  <525A6831.5070402@gmail.com> <l3gc7e$c91$1@ger.gmane.org> <20131014133953.58f74659@gumby.homeunix.com> <525C1D1C.9050708@gmail.com> <CA+tpaK2Pr2po2cQ1yanQK9+wLp77SYqYHjxiXaU5FfXwHrkGow@mail.gmail.com> <CAFYkXjn-1wTJcQ4a_fyXCvwh9ukt3+djM2qsMeaH1HhVJNvhiA@mail.gmail.com>

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On 10/14/2013 12:50 PM, CeDeROM wrote:
> On Mon, Oct 14, 2013 at 6:47 PM, Adam Vande More <amvandemore@gmail.com> wrote:
>> On Mon, Oct 14, 2013 at 11:34 AM, David Demelier
>> <demelier.david@gmail.com>wrote:
>>> Why? SU+J is enabled by default. Isn't the purpose of a journaled file
>>> system to ensure that any bad shutdown will protect data?
>> As already stated, those measures are to preserve fs integrity eg meta data
>> is in sync.  It doesn't ensure that all the outstanding writes are
>> committed to disk in the event of a power outage.
> Then why random files gets damaged as well even they are not
> accessed/written on power loss? :-)
Random files can be affected because the sectors of the hard disk 
containing the directory entries for those files, not the file data 
itself, may be damaged (ie: the directory was in the process of being 
written OR the pointer to that SECTOR was in the process of being written).

It doesn't mean a file was in active use, just that a chunk of the disk 
with data relevant to that file was. Keep in mind, one sector of disk 
may have data for a dozen files in it (or more). Damage doesn't have to 
occur because a given file was in use at the time of a crash.

If your power grid is prone to failures or blips, I strongly suggest 
investing in a UPS.

Brad




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