linux-ext4 - Re: [patch] ext2/3: document conditions when reliable operation is possible

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Date:	Tue, 25 Aug 2009 16:48:45 -0700 (PDT)
From:	david@...g.hm
To:	Ric Wheeler <rwheeler@...hat.com>
cc:	Pavel Machek <pavel@....cz>, Theodore Tso <tytso@....edu>,
	Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	Rob Landley <rob@...dley.net>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, corbet@....net
Subject: Re: [patch] ext2/3: document conditions when reliable operation is
 possible

On Tue, 25 Aug 2009, Ric Wheeler wrote:

> On 08/25/2009 07:26 PM, Pavel Machek wrote:
>> 
>>>>> Basically, any file system (Linux, windows, OSX, etc) that writes into
>>>>> the page cache will lose data when you hot unplug its storage. End of
>>>>> story, don't do it!
>>>> 
>>>> No, not ext3 on SATA disk with barriers on and proper use of
>>>> fsync(). I actually tested that.
>>>> 
>>>> Yes, I should be able to hotunplug SATA drives and expect the data
>>>> that was fsync-ed to be there.
>>> 
>>> You can and will lose data (even after fsync) with any type of storage at
>>> some rate. What you are missing here is that data loss needs to be
>>> measured in hard numbers - say percentage of installed boxes that have
>>> config X that lose data.
>> 
>> I'm talking "by design" here.
>> 
>> I will lose data even on SATA drive that is properly powered on if I
>> wait 5 years.
>> 
>
> You are dead wrong.
>
> For RAID5 arrays, you assume that you have a hard failure and a power outage 
> before you can rebuild the RAID (order of hours at full tilt).

and that the power outage causes a corrupted write.

>>> I can promise you that hot unplugging and replugging a S-ATA drive will
>>> also lose you data if you are actively writing to it (ext2, 3, whatever).
>> 
>> I can promise you that running S-ATA drive will also lose you data,
>> even if you are not actively writing to it. Just wait 10 years; so
>> what is your point?
>
> I lost a s-ata drive 24 hours after installing it in a new box. If I had MD5 
> RAID5, I would not have lost any.

me to, in fact just after I copied data from a raid array to it so that I 
could rebuild the raid array differently :-(

David Lang
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