Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Tejun Heo <htejun@gmail.com>]

Ric Wheeler wrote:
>> Don't those thingies usually have NV cache or backed by battery such
>> that ORDERED_DRAIN is enough?
> 
> All of the high end arrays have non-volatile cache (read, on power loss,
> it is a promise that it will get all of your data out to permanent
> storage). You don't need to ask this kind of array to drain the cache.
> In fact, it might just ignore you if you send it that kind of request ;-)
> 
> The size of the NV cache can run from a few gigabytes up to hundreds of
> gigabytes, so you really don't want to invoke cache flushes here if you
> can avoid it.
> 
> For this class of device, you can get the required in order completion
> and data integrity semantics as long as we send the IO's to the device
> in the correct order.

Thanks for clarification.

>> The problem is that the interface between the host and a storage device
>> (ATA or SCSI) is not built to communicate that kind of information
>> (grouped flush, relaxed ordering...).  I think battery backed
>> ORDERED_DRAIN combined with fine-grained host queue flush would be
>> pretty good.  It doesn't require some fancy new interface which isn't
>> gonna be used widely anyway and can achieve most of performance gain if
>> the storage plays it smart.
> 
> I am not really sure that you need this ORDERED_DRAIN for big arrays...

ORDERED_DRAIN is to properly order requests from host request queue
(elevator/iosched).  We can make it finer-grained but we do need to put
some ordering restrictions.

-- 
tejun