Andre Merzky wrote:
consider following use case for remote IO. Given a large binary 2D field on a remote host, the client wans to access a 2D sub portion of that field. Dependend on the remote file layout, that requires usually more than one read operation, since the standard read (offset, length) is agnostic to the 2D layout.
For more complex operations (subsampling, get a piece of a jpg file), the number of remote operations grow very fast. Latency then stringly discourages that type of remote IO.
For that reason, I think that the remote file IO as specified by SAGA's Strawman as is will only be usable for a limited and trivial set of remote I/O use cases.
There are three (basic) approaches:
A) get the whole thing, and do ops locally Pro: - one remote op, - simple logic - remote side doesn't need to know about file structure - easily implementable on application level Con: - getting the header info of a 1GB data file comes with, well, some overhead ;-)
B) clustering of calls: do many reads, but send them as a single request. Pro: - transparent to application - efficient Con: - need to know about dependencies of reads (a header read needed to determine size of field), or included explicite 'flushes' - need a protocol to support that - the remote side needs to support that
C) data specific remote ops: send a high level command, and get exactly what you want. Pro: - most efficient Con: - need a protocol to support that - the remote side needs to support that _specific_ command
The last approach (C) is what I have best experiences with. Also, that is what GridFTP as a common file access protocol supports via ERET/ESTO operations.
I want to propose to include a C-like extension to the File API of the strawman, which basically maps well to GridFTP, but should also map to other implementations of C.
Agreed here.
That extension would look like:
void lsEModes (out array<string,1> emodes ); void eWrite (in string emode, in string spec, in string buffer out long len_out ); void eRead (in string emode, in string spec, out string buffer, out long len_out );
- hooks for gridftp-like opaque ERET/ESTO features - spec: string for pattern as in GridFTP's ESTO/ERET - emode: string for ident. as in GridFTP's ESTO/ERET
EMode: a specific remote I/O command supported lsEModes: list the EModes available in this implementation eRead/eWrite: read/write data according to the emode spec
Example (in perl for brevity):
my $file = SAGA::File new ("http://www.google.com/intl/en/images/logo.gif"); my @emodes = $file->lsEModes ();
if ( grep (/^jpeg_block$/, @emodes) ) { my ($buff, $len) = file.eRead ("jpeg_block", "22x4+7+8"); }
I would discourage support for B, since I do not know any protocoll supporting that approach efficiently, and also it needs approximately the same infrastructure setup as C.
As A is easily implementable on application level, or within any SAGA implementation, there is no need for support on API level -- however, A is insufficient for all but some trivial cases.
This approach is very generic on the API level (that's good) but requires exact agreement on the used command syntax for the client and the server, which may get problematic. If we go this route we will definitely end up specifying at least a minimal command subset to be supported by the eRead/eWrite commands. I simply fear we'll have the same problems we have with the GAT today. The GAT API is in principle usable in a broad range of use cases based on a generic API. The genericity is ensured by using key/value tables in the API itself, allowing quick adaptation to any concrete need. The problem is the missing specification of these key/value pairs which makes it difficult to achieve reusability. Regards Hartmut