Re: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible)

[Ric Wheeler <rwheeler@redhat.com>]

On 08/31/2009 11:50 AM, david@lang.hm wrote:
> On Mon, 31 Aug 2009, Ric Wheeler wrote:
>
>> 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).
>
> I've done this when the hardware raid only suppored raid 5 but I wanted
> raid 6. I've also done it when I had enough disks to need more than one
> hardware raid card to talk to them all, but wanted one logical drive for
> the system.
>
>> 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....
>
> most people are not willing to live with unbuffered write performance.
> they care about their data, but they also care about performance, and
> since performance is what they see on an ongong basis, they tend to care
> more about performance.
>
> given that we don't even have barriers enabled by default on ext3 due to
> the performance hit, what makes you think that disabling buffers
> entirely is going to be acceptable to people?
>
> David Lang

We do (and have for a number of years) enable barriers by default for XFS and 
reiserfs. In SLES, ext3 has default barriers as well.

Ric