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SubjectRe: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible)
On 08/31/2009 09:16 AM, Christoph Hellwig wrote:
> On Mon, Aug 31, 2009 at 09:15:27AM -0400, Ric Wheeler wrote:
>>> While most common filesystem do have barrier support it is:
>>>
>>> - not actually enabled for the two most common filesystems
>>> - the support for write barriers an cache flushing tends to be buggy
>>> all over our software stack,
>>>
>>
>> Or just missing - I think that MD5/6 simply drop the requests at present.
>>
>> I wonder if it would be worth having MD probe for write cache enabled&
>> warn if barriers are not supported?
>
> In my opinion even that is too weak. We know how to control the cache
> settings on all common disks (that is scsi and ata), so we should always
> disable the write cache unless we know that the whole stack (filesystem,
> raid, volume managers) supports barriers. And even then we should make
> sure the filesystems does actually use barriers everywhere that's needed
> which failed at for years.
>

I was thinking about that as well. Having us disable the write cache when we
know it is not supported (like in the MD5 case) would certainly be *much* safer
for almost everyone.

We would need to have a way to override the write cache disabling for people who
either know that they have a non-volatile write cache (unlikely as it would
probably be to put MD5 on top of a hardware RAID/external array, but some of the
new SSD's claim to have non-volatile write cache).

It would also be very useful to have all of our top tier file systems enable
barriers by default, provide consistent barrier on/off mount options and log a
nice warning when not enabled....

ric



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