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Re^3: Combining Ultra-Dynamic Files to Avoid Clustering (Ideas?)( A DB won't help)

by BrowserUk (Pope)
on Jul 26, 2004 at 00:22 UTC ( #377323=note: print w/replies, xml ) Need Help??


in reply to Re^2: Combining Ultra-Dynamic Files to Avoid Clustering (Ideas?)( A DB won't help)
in thread Combining Ultra-Dynamic Files to Avoid Clustering (Ideas?)

As described by the OP, there are 1,000,000(+) binary files containing (a variable number of) 4-byte integers often less than 1kb, and usually less than 4kb. Assuming an average of 2kb/512 integers per file that gives 2,048*1,000,000 = 1.9 GB. The aim was to save 'wasted disc space' due to clustersize round-up.

Any DB scheme that uses a single table and 2x 4-byte integer indices per number will require (minimum) 12 * 512 * 1,000,000 = 5.7 GB.

The extra space is required because the two indices, fileno & itemno(position) are implicite in the original scheme, but must be explicit in the 'one table/one number per tuple' scheme.

The other alternative I posed was to store the each file (1..1024 4-byte integers) from the filesystem scheme as LONGBLOBs thereby packing 1 file per tuple in the single table. Often BLOBS are stored as fixed length records, each occupying the maximum record size allowed regradless of the length actually stored.

Even when they are stored as LONGVARBINARY (4-byte length+length bytes) they are not stored in the main table file, but in separate file with a 4-byte placeholder/pointer into the ancillary file. That's at least 12-bytes/file (fileno, pointer, length) * 1,000,000 extra bytes that need to be stored on disc somewhere. Any savings made through avoiding cluster round-up by packing the variable length records into a single file are mostly lost here and in the main table file.

In addition as the OP pointed out, this sceme requires that each 'file' record be queried, appended to, and then re-written for each number added. A costly process relative to appending to the end of a named file.

It's often forgotten that ultimately data stored in a database end's up in the filesystem (in most cases). Of course, in a corporate environment, that disc space may belong to someone else's budget and is therefore not a concern :) But if the aim is to save disc space (which may or may not be a legitimate concern--we don't know the OP's situation. Embedded systems?), then a DB won't help.


Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
"Memory, processor, disk in that order on the hardware side. Algorithm, algoritm, algorithm on the code side." - tachyon
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