[Go-essp-tech] Reasoning for the use of symbolic links in drslib

Tue Sep 20 15:35:32 MDT 2011

Hi All,

Lots of good discussion here and sorry I've been keeping quiet.  I want to remind ourselves of the requirements I laid out in the wiki page

1. It should allow data from multiple versions to be kept on disk simultaneously.
2. It should avoid storing multiple copies of files that are present in more than one version.
3. It should be straightforward to copy dataset changes (i.e. differences between versions) between nodes to allow efficient replication.
4. It should rely only on the filesystem so that generic tools like FTP could be used to expose the structure if necessary.

In my view we should address these directly.  Are they needed?  Which are the most important?

Gavin said about catalogs
> you can quickly and easily inspect catalog_v1 and catalog_v2 to find what the changes are.
> This all answers the question of "WHAT" (to download)... the other question of "HOW" is a different, but related story.
> The trick is to not conflate the two issues which is what filesystem discussions do. .

But THREDDS conflates the two as well!  A THREDDS catalog contains descriptions of service endpoints that are not independent of the node serving the data (the "HOW").  Maybe we should have developed a true catalog format but that is not where we are now.  The replication client use THREDDS catalogs in this way but when I last looked it was completely unaware of versions -- i.e. it won't help with #3.

I don't see how Gavin's point addresses any of the requirements above.  Even if we ditch #4, which I expect Gavin would argue for, it doesn't directly solve the problem for #1-#3 either.

Briefly on some other points that have been made...

Balaji, some archive tools maybe can detect 2 paths pointing to the same filesystem inode but both Estani and I have enquired with our backup people and they say hard links must be avoided.  I am happy to include a hard-linking option in drslib though.  I've created a bugzilla ticket for it.

Karl, I think putting real files in "latest" is equivalent to putting real files in the latest "vYYYYMMDD" directory.  The directories can be renamed trivially on upgrade but you still have the same problems as the wiki page says.

I'm sure there were other points but I've lost track.  Checksums will have to wait for another email.

Cheers,
Stephen.

---
Stephen Pascoe  +44 (0)1235 445980
Centre of Environmental Data Archival
STFC Rutherford Appleton Laboratory, Harwell Oxford, Didcot OX11 0QX, UK

From: go-essp-tech-bounces at ucar.edu [mailto:go-essp-tech-bounces at ucar.edu] On Behalf Of Gavin M. Bell
Sent: 20 September 2011 17:26
To: Kettleborough, Jamie
Cc: go-essp-tech at ucar.edu; esg-node-dev at lists.llnl.gov
Subject: Re: [Go-essp-tech] Reasoning for the use of symbolic links in drslib

Jamie and friends.

You've answered your own questions :-)...
It is the catalog where these checksums (and other features) are recorded.
And thus using the catalog you can see what has changed.
There is a new catalog for every version of a dataset. Given that...
you can quickly and easily inspect catalog_v1 and catalog_v2 to find what the changes are.
This all answers the question of "WHAT" (to download)... the other question of "HOW" is a different, but related story.
The trick is to not conflate the two issues which is what filesystem discussions do.  When talking about filesystems you are stipulating the what but implicitly conflating the HOW because you are implicitly designing for tools that intrinsically use the filesystem.  It is a muddying of the waters that doesn't separate the two concerns.  We need to deal with these two concepts independently in a way that does not  limit the system or cause undo burden on institutions by requiring a rigid structure.

As I mentioned... it's not the filesystem we need to look at... it's the catalogs.

just my $0.02 - I'll stop flogging this particular horse... but I hope I have done a better job expressing the issues and where the solution lies (IMHO).

On 9/20/11 8:14 AM, Kettleborough, Jamie wrote:

Hello Balaji,

I agree - getting all nodes to make the checksums available would be a

good thing.  It gives you both the data integrity check on download, and

the ability to see what files really have changed from one publication

version to the next.

I don't know how hard it is to do this, particularly for data that is

already published.

Jamie

-----Original Message-----

From: V. Balaji [mailto:V.Balaji at noaa.gov]

Sent: 20 September 2011 16:01

To: Kettleborough, Jamie

Cc: Karl Taylor; go-essp-tech at ucar.edu<mailto:go-essp-tech at ucar.edu>; esg-node-dev at lists.llnl.gov<mailto:esg-node-dev at lists.llnl.gov>

Subject: Re: [Go-essp-tech] Reasoning for the use of symbolic

links in drslib

If nodes can currently choose to record checksums or not, I'd

strongly recommend this be a non-optional requirement.. how

could anyone download any data with confidence without being

able to checksum?

You can of course check timestamps and filesizes and so on,

but you have to consider those optimizations... a fast option

for the less paranoid to avoid the sum computation, which has

to be the gold standard.

"Trust but checksum".

Kettleborough, Jamie writes:

Hello Karl, everyone,

   For replicating the latest version, I agree that your alternate

structure poses difficulties (but it seems like there must

be a way to

smartly determine whether the file you already have a file

and simply

need to move it, rather than bring it over again).

Doesn't every user (not just the replication system) have

this problem:

they want to know what files have changed (or not changed) at a new

publication version.  No one wants to be using band width

or storage

space to fetch and store files they already have.  How is a user

expected to know what has really changed?  Estani mentions

check sums

- OK, but I don't think all nodes expose them (is this

right?).  You

may try to infer from modification dates (not sure, I

haven't look at

them that closely).  You may try to infer from the

TRACKING_ID - but

I'm not sure how reliable this is (I can imagine scenarios where

different files share the same TRACKING_ID - e.g. if they have been

modified with an nco tool).

Is there a recommended method for users to understand what *files*

have actually changed when a new publication version appears?

Thanks,

Jamie

--

V. Balaji                               Office:  +1-609-452-6516

Head, Modeling Systems Group, GFDL      Home:    +1-212-253-6662

Princeton University                    Email: v.balaji at noaa.gov<mailto:v.balaji at noaa.gov>

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Gavin M. Bell

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