linux-ext4 - Re: [patch] document flash/RAID dangers

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Date:	Tue, 25 Aug 2009 20:50:03 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Pavel Machek <pavel@....cz>
CC:	david@...g.hm, 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] document flash/RAID dangers

On 08/25/2009 08:44 PM, Pavel Machek wrote:
>
>>>>> THESE devices have the property of potentially corrupting blocks being
>>>>> written at the time of the power failure,
>>>>
>>>> this is true of all devices
>>>
>>> Actually I don't think so. I believe SATA disks do not corrupt even
>>> the sector they are writing to -- they just have big enough
>>> capacitors. And yes I believe ext3 depends on that.
>>
>> Pavel, no S-ATA drive has capacitors to hold up during a power failure
>> (or even enough power to destage their write cache). I know this from
>> direct, personal knowledge having built RAID boxes at EMC for years. In
>> fact, almost all RAID boxes require that the write cache be hardwired to
>> off when used in their arrays.
>
> I never claimed they have enough power to flush entire cache -- read
> the paragraph again. I do believe the disks have enough capacitors to
> finish writing single sector, and I do believe ext3 depends on that.
>
> 									Pavel

Some scary terms that drive people mention (and measure):

"high fly writes"
"over powered seeks"
"adjacent tack erasure"

If you do get a partial track written, the data integrity bits that the data is 
embedded in will flag it as invalid and give you and IO error on the next read. 
Note that the damage is not persistent, it will get repaired (in place) on the 
next write to that sector.

Also it is worth noting that ext2/3/4 write file system "blocks" not single 
sectors. Each ext3 IO is 8 distinct disk sector writes and those can span tracks 
on a drive which require a seek which all consume power.

On power loss, a disk will immediately park the heads...

ric

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