Date: Sun, 4 Jan 2009 16:16:30 -0800 (PST)
From: david@lang.hm
To: Pavel Machek <pavel@suse.cz>
cc: Rob Landley <rob@landley.net>, kernel list <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@osdl.org>, tytso@mit.edu, mtk.manpages@gmail.com,
       rdunlap@xenotime.net, linux-doc@vger.kernel.org
Subject: Re: document ext3 requirements
In-Reply-To: <20090104225545.GF1913@elf.ucw.cz>
Message-ID: <alpine.DEB.1.10.0901041615560.25545@asgard.lang.hm>
References: <20090103123813.GA1512@ucw.cz> <200901041349.49906.rob@landley.net> <20090104225545.GF1913@elf.ucw.cz>
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On Sun, 4 Jan 2009, Pavel Machek wrote:

> On Sun 2009-01-04 13:49:49, Rob Landley wrote:
>> On Saturday 03 January 2009 06:38:15 Pavel Machek wrote:
>>> +Ext3 expects disk/storage subsystem to behave sanely. On sanely
>>> +behaving disk subsystem, data that have been successfully synced will
>>> +stay on the disk. Sane means:
>>> +
>>> +* writes to media never fail. Even if disk returns error condition during
>>> +  write, ext3 can't handle that correctly, because success on fsync was
>>> already +  returned when data hit the journal.
>>> +
>>> +	   (Fortunately writes failing are very uncommon on disks, as they
>>> +	   have spare sectors they use when write fails.)
>>> +
>>> +* either whole sector is correctly written or nothing is written during
>>> +  powerfail.
>>> +
>>> +	   (Unfortuantely, none of the cheap USB/SD flash cards I seen do behave
>>> +	   like this, and are unsuitable for ext3.
>>
>> Want to document the granularity issues with flash, while you're at it?
>>
>> An inherent problem with using flash as a normal block device is that the
>> flash erase size is bigger than most filesystem sector sizes.  So when you
>> request a write, it may erase and rewrite the next 64k, 128k, or even a couple
>> megabytes on the really _big_ ones.
>>
>> If you lose power in the middle of that, ext3 won't notice that data in the
>> "sectors" _after_ the one your were trying to write to got trashed.
>>
>> The flash filesystems take this into account as part of their wear levelling
>> stuff (they normally copy the entire chunk into a new chunk, leaving the old
>> one in place until it's no longer needed), but they need to query the device
>> to get the erase granularity in order to do that, which is why they don't work
>> on non-flash block devices.
>
> Is there linux filesystem that can handle that? I know jffs2, but
> that's unsuitable for stuff like USB thumb drives, right?
>
> Does this sound like a fair summary?
>
> Sector writes are atomic (ATOMIC-SECTORS)
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Either whole sector is correctly written or nothing is written during
> powerfail.
>
>        Unfortuantely, none of the cheap USB/SD flash cards I seen do
>        behave like this, and are unsuitable for all linux filesystems
>        I know.
>
>                An inherent problem with using flash as a normal block
>                device is that the flash erase size is bigger than
>                most filesystem sector sizes.  So when you request a
>                write, it may erase and rewrite the next 64k, 128k, or
>                even a couple megabytes on the really _big_ ones.
>
>                If you lose power in the middle of that, filesystem
>                won't notice that data in the "sectors" _after_ the
>                one your were trying to write to got trashed.

around, not after. the block you are reading could be in the middle or at 
the end of an eraseblock.

David Lang
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