Alan Louis Scheinine wrote:
There is a particular kind of application, single-client
and serial process, for which a striped file system using
RAM disk would be very useful. Consider reading small
blocks at random locations on a hard disk. The latency
of the HDD could be a few milliseconds. Adding more HDD's
does not solve the problem, unlike an application based on
streaming. Adding more disks and parallelizing the program
could be a solution but sometimes there is no time
to parallelize the program.
A possible solution is RAM disk. But if we put, for example,
64 GB of RAM on a single computer then that computer becomes
specialized and expensive, whereas the need for a huge
amount of RAM may be only temporary. An alternative is to
use a cluster of nodes, a typical Beowulf cluster. For example,
using a striped file system over 16 nodes where each node has 4 GB
of RAM. Each node would have a normal amount of RAM and yet
could provide the aggregate storage of 64 GB when the need arises.
While we have not yet created this configuration, I suppose
that Gbit Ethernet could provide 100 microsecond latency and
Infiniband or Myrinet could provide 10 microsecond latency.
Much, much less than the seek time of a HDD.
The idea is so simple that I imagine it has already been done.
I would be interested in learning from other sites that have
used this method with a file system such as Lustre, PVFS2 or
another.
best regards,
Alan Scheinine
We do this at SiCortex using Lustre as the parallel filesystem, with
tmpfs as the "backing store". The
marketing folks call it "FabriCache". The folks at Argonne are working
on doing the same thing
with PVFS2 on the SiCortex machine they have. It works well for us, and
should work pretty
well on traditional cluster hardware as well.
I think there is room to improve the idea as well, perhaps by using some
kind of journalling to
disk, with coherent timestamps, so the striped journals could be
correlated into a consistent
view, but we haven't worked on that.
-Larry
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