Hi Gregory,
On 12/11/13 10:13, Gregory Farnum wrote:
On Mon, Nov 11, 2013 at 3:04 AM, John Spray <[email protected]> wrote:
This is a really useful summary from Malcolm.
In addition to the coordinator/copytool interface, there is the question of
where the policy engine gets its data from. Lustre has the MDS changelog,
which Robinhood uses to replicate metadata into its MySQL database with all
the indices that it wants.
On Sun, Nov 10, 2013 at 11:17 PM, Malcolm Haak <[email protected]> wrote:
So there aren't really any hooks in that exports are triggered by the policy
engine after a scan of the metadata, and the recalls are triggered when caps
are requested on offline files
Wait, is the HSM using a changelog or is it just scanning the full
filesystem tree? Scanning the whole tree seems awfully expensive.
While I can't speak at length about the LustreHSM, it may just use
incremental updates to its SQL database via metadata logs, I do know
that filesystem scans are done regularly in other HSM solutions. I also
know that the scan is multi-threaded and when backed by decent disks
does not take an excessive amount of time.
I don't know if CephFS MDS currently has a similar interface.
Well, the MDSes each have their journal of course, but more than that
we can stick whatever we want into the metadata and expose it via
virtual xattrs or whatever else.
-Greg
Software Engineer #42 @ http://inktank.com | http://ceph.com
John
On Sun, Nov 10, 2013 at 11:17 PM, Malcolm Haak <[email protected]> wrote:
Hi All,
If you are talking specifically about Lustre HSM, its really an interface to
add HSM functionality by leveraging existing HSM's (DMF for example)
So with Lustre HSM you have a policy engine that triggers the migrations out of
the filesystem. Rules are based around size, last accessed and target state
(online, dual and offline).
There is a 'coordinator' process involved here as well, it (from what I
understand) runs on MDS nodes. It handles the interaction with the copytool.
The copytool is provided by the HSM solution you are acutally using.
For recalls when caps are aquired on the MDS for an exported file the
resposible MSD contacts the coordinator, which in-turn uses the copytool to
pull the required file out of the HSM.
In the Lustre HSM, the objects that make up a file are all recalled and the
file, not the objects, are handed to the HSM.
For Lustre all it needs to keep track of is the current state of the file and
the correct ID to reqest from the HSM. This is done inside the normal metadata
storage.
So there aren't really any hooks in that exports are triggered by the policy
engine after a scan of the metadata, and the recalls are triggered when caps
are requested on offline files. Then its just standard POSIX blocking until the
file is available.
Most of the state and ID stuff could be stored as XATTRS in cephfs. I'm not as
sure how to do it for other things but as long as you could store some kind of
extended metadata about whole objects, it could use the same interfaces as well.
Hope that was acutually helpful and not just an obvious rehash...
Regards
Malcolm Haak
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