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



On Fri, 1 Jun 2007, Tejun Heo wrote:

but one
thing we should bear in mind is that harddisks don't have humongous
caches or very smart controller / instruction set.  No matter how
relaxed interface the block layer provides, in the end, it just has to
issue whole-sale FLUSH CACHE on the device to guarantee data ordering on
the media.

if you are talking about individual drives you may be right for the moment (but 16M cache on drives is a _lot_ larger then people imagined would be there a few years ago)

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.

David Lang


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