Re: Initial filesystem design synopsis.

["Thomas E. Spanjaard" <tgen@xxxxxxxxxxxxx>]

Matthew Dillon wrote:
> Segmentation:
>
>     The physical storage backing a filesystem is broken up into large
>     1MB-4GB segments (64MB is a typical value).  Each segment is
>     self-identifying and contains its own header, data table, and record
>     table.  The operating system glues together filesystems and determines
>     availability based on the segments it finds.

I think the more common term for this kind of thing is 'allocation group'.

>     - The data table consists of pure data, laid out linearly in the forward
>       direction within the segment.   Data blocks are variable-sized entities
>       containing pure data, with no other identifying information, suitable
>       for direct DMA.  The segment header has a simple append index for
>       the data table.

And 'extent' for the variable-sized entities :).

>     - The record table consists of fixed-sized records and a reference to
>       data in the data table.  The record table is built backwards from
>       the end of the segment.

Doesn't this prepending stuff incur a significant performance penalty 
for operations that walk the record table in a chronological/otherwise 
'fifo' ordered fashion?

>     Record destruction creates holes in both the data table and the record
>     table.  Any holes adjacent to the data table append point or the record
>     table prepend point are immediately recovered by adjusting the 
>     appropriate indices in the segment header.  The operating system may
>     cache a record of non-adjacent holes (in memory) and reuse the space,
>     and can also generate an in-memory index of available holes on the
>     fly when space is very tight (which requires scanning the record table),
>     but otherwise the recovery of any space not adjacent to the data table
>     append point requires a performance reorganization of the segment.

I think these lists/trees should be kept sorted, at least on-disk for 
performance reasons (random reads/writes on rotational media is a bummer 
given current seek times).

Generally, I can't help but feel that the clustering/replication stuff 
needs to be separate from the 'actual on-disk' filesystem.

Cheers,
--
        Thomas E. Spanjaard
        tgen@netphreax.net

Attachment: signature.asc
Description: OpenPGP digital signature