Multi-file databases were implemente to handle databases larger than the
disk or file systen size. Disks have gotten a great deal larger, but so
have databases, so obviously something has to give to support your 7 TB
database. I sense an interesting design opportunity.
On Wednesday, March 2, 2016, Dmitry Yemanov <firebi...@yandex.ru> wrote:
>
> Historically, Firebird databases consist of a sequential set of pages of
> the fixed size (4-16KB currently). This page set is distributed across
> one (usually) or multiple files (*) The page number initially was SLONG,
> now it's ULONG. So the theoretically possible maximum database size is
> currently limited to 2^32 * 16KB.
>
> When we speak about tablespaces, it usually means that the database
> consists of multiple files and different database object are stored in
> different files. Each such file is named within a database and called a
> tablespace. And each tablespace has its own page set and page numbering.
>
> A typical usage pattern is that tablespaces are used to separate table
> data from indices (and logs from the rest of the database) and thus
> allow better concurrent performance due to parallel I/O. Often it's
> argued that RAIDs now handle the same job and maybe even better. For
> many usage cases - maybe. But I'm pretty sure that the opposite cases
> are also possible, when a carefully designed partitioning could
> outperform automatic RAID data management.
>
> Another usage case could be extending the database size beyond the
> current limits. The limit is 64TB, the biggest FB database I known is
> 7TB. Not that far, I'd say. The limit may be shifted with even larger
> page sizes, but it has its drawbacks as well.
>
> Someone may think about per-tablespace physical backups and other
> possible usage cases. So I'm sure this feature is something to be at
> least considered. From another side, tablespaces complicate maintenance,
> so it's something more for enterprise users rather than for common FB
> users.
>
> Now back to the code. During the Firebird development, we have
> introduced a concept of "page spaces", represented with a PageSpace
> class. It implements a two-level numbering for database pages: pagespace
> ID + page number. The whole engine is aware of that. Default pagespace
> (ID == 0, IIRC) is reserved to the database file(s). Non-zero pagespace
> IDs are currently used for GTTs (global temporary tables) that have
> their data/indices stored in temporary files.
>
> Technically, nothing prevents us from declaring named tablespaces via
> DDL (CREATE/ALTER/DROP TABLESPACE?), storing their definitions inside
> the metadata (RDB$TABLESPACES table?), allocating some pagespace ID to
> the every tablespace, and allowing to specify a tablespace when creating
> database objects (tables, indices, what else?).
>
> Of course, there are more details hidden that must be addressed. Maybe
> I'm missing something in my review. But I think this thread could be a
> good starting point for discussion.
>
> Others are welcome to contribute their thoughts.
>
> (*) My personal opinion is that legacy multi-file databases must die,
> preferrably in FB4. They make zero sense in modern filesystems. They're
> not supported by nbackup. They may complicate implementation of
> tablespaces. Anyone here still using multi-file databases?
>
>
> Dmitry
>
>
>
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--
Jim Starkey
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