lkml.org 
[lkml]   [2009]   [Aug]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [patch] document flash/RAID dangers
On Wed, 26 Aug 2009, 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.

keep in mind that in a powerfail situation the data being sent to the
drive may be corrupt (the ram gets flaky while a DMA to the drive copies
the bad data to the drive, which writes it before the power loss gets bad
enough for the drive to decide there is a problem and shutdown)

you just plain cannot count on writes that are in flight when a powerfail
happens to do predictable things, let alone what you consider sane or
proper.

David Lang


\
 
 \ /
  Last update: 2009-08-26 03:23    [W:0.195 / U:1.420 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site