The best of both worlds would certainly be a central, fast storage
filesystem, coupled with a hierarchical storage management system.
I'm not sure - is there some clear indication that one level of storage is
not good enough?
I guess it strongly depends on your workload and applications. If your users
tend to keep all their files for long-term purposes, as Bogdan Costescu
pertinently described earlier, it might make sense to transparently free up
the fast centralized filesystem and move the unused-at-the-moment-but-still-
crucially-important files to a slower, farther filesystem (or tapes).
no: my point concerned only whether there is a meaningful distinction
between what you call fast and slow storage. from a hardware perspective,
there is can be differences in latency, though probably not noticed by
anything other than benchmarks or quite specialized apps. there can also
be aggregate throughput differences: an OST on QDR IB vs Gb.
so, an open question: do HPC people still buy high-end disks (small FC/SAS
disks, usually also 10-15k)? we're putting them in our next MDS, but
certainly not anywhere else.
another question: do HPC people still buy tapes? I'm personally opposed,
but some apparently reasonable people find them comforting (oddly, because
they can to some limited extent be taken offline.)
this seems like a bad design to me. I would think (and I'm reasonably
familiar with Lustre, though not an internals expert) that if you're going
to touch Lustre interfaces at all, you should simply add cheaper,
higher-density OSTs, and make more intelligent placement/migration
heuristics.
In Lustre, that would be done through OST pools. Eh, isn't this also a feature
CEA contributed to? :)
is there a documented, programmatic lustre interface to control OST placement
(other than lfs setstripe)? such an interface would also need a
high-performance way to query the MDS directly (not through the mounted
FS, which is too slow for any seriously large FS.)
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