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Re: [dm-devel] [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush



On 2010-08-23 15:58, Ric Wheeler wrote:
> On 08/23/2010 08:48 AM, Christoph Hellwig wrote:
>> On Mon, Aug 23, 2010 at 02:30:33PM +0200, Tejun Heo wrote:
>>> It might be useful to give several example configurations with
>>> different cache configurations.  I don't have much experience with
>>> battery backed arrays but aren't they suppose to report write through
>>> cache automatically?
>>
>> They usually do.  I have one that doesn't, but SYNCHRONIZE CACHE on
>> it is so fast that it effectively must be a no-op.
>>
> 
> Arrays are not a problem in general - they normally have internally, redundant 
> batteries to hold up the cache.
> 
> The issue is when you have an internal hardware RAID card with a large cache. 
> Those cards sit in your server and the batteries on the card protect its 
> internal cache, but do not have the capacity to hold up the drives behind it.
> 
> Normally, those drives should have their write cache disabled, but sometimes 
> (especially with S-ATA disks) this is not done.

The problem purely exists on arrays that report write back cache enabled
AND don't implement SYNC_CACHE as a noop. Do any of them exist, or are
they purely urban legend?

-- 
Jens Axboe


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