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





Tejun Heo wrote:
[ cc'ing Ric Wheeler for storage array thingie.  Hi, whole thread is at
http://thread.gmane.org/gmane.linux.kernel.device-mapper.devel/3344 ]

I am actually on the list, just really, really far behind in the thread ;-)


Hello,

david lang hm wrote:
but when you consider the self-contained disk arrays it's an entirely
different story. you can easily have a few gig of cache and a complete
OS pretending to be a single drive as far as you are concerned.

and the price of such devices is plummeting (in large part thanks to
Linux moving into this space), you can now readily buy a 10TB array for
$10k that looks like a single drive.

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.


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.

Thanks.


I am not really sure that you need this ORDERED_DRAIN for big arrays...

ric


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