
The Five-Minute Rule 20 Years Later - ehsanul
http://cacm.acm.org/magazines/2009/7/32091-the-five-minute-rule-20-years-later/fulltext
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herf
ZFS handles the three-tiered model well (with the L2ARC implementation). Also,
the authors of this paper didn't consider the "write-limited" implementation
that the ZFS authors use: they write at a fixed rate to SSD (e.g., 8MB/sec)
and pull entries near the end of the RAM-based cache (i.e. the ones that are
about to expire). Still, quite a great paper.

[http://www.sun.com/emrkt/openstorage/0309engineers.html?cid=...](http://www.sun.com/emrkt/openstorage/0309engineers.html?cid=e8017)

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harpastum
So, can anyone explain why the three-tiered memory architecture described here
(RAM, flash, HD) isn't widely used in computing today? It seems to me like
32GB/64GB of flash memory in between disk and RAM would dramatically increase
performance of virtual memory, disk access, etc. without increasing cost very
much.

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nzmsv
Hardware costs won't increase, you are right. But the costs to redesign the
software to use an extra caching layer are going to be significant. Also, the
current situation with flash memory being expensive won't last long, and
moving to pure solid-state storage is still better than this 3-tier
architecture. So, it's a large investment into a solution with only a couple
years of lifespan.

Of course, this is probably wrong, and there is a startup somewhere doing just
this, and they will be rich :)

~~~
neilc
There are lots of people in research looking at incorporating SSDs into the
storage hierarchy in various ways. Not sure how much of this is going to be
productized in the short term, but I wouldn't be surprised to see, say, a new
release of a commercial DBMS that can use SSDs for some of the buffer cache.

 _Also, the current situation with flash memory being expensive won't last
long, and moving to pure solid-state storage is still better than this 3-tier
architecture._

For scan-intensive workloads (e.g. datawarehousing), I'd expect to see
magnetic disks continue to be used for quite some time. Buying a petabyte of
SSD would be _expensive_.

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BearOfNH
The author was careful to restrict his comments to SLC drives, which have
reasonably long lifetimes.

The newer, cheaper MLC technology flash drives have _far_ less useful
lifetime, somewhere in the vicinity of 5,000-10,000 erase cycles. These drives
just won't cut it for usages defined here, though they will work fine in your
cell phone.

See (e.g.) <http://en.wikipedia.org/wiki/Flash_memory#Write_Endurance>

