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1 | 1 | src/backend/storage/smgr/README |
2 | 2 |
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3 | | -StorageManager |
4 | | -=============== |
| 3 | +StorageManagers |
| 4 | +================ |
5 | 5 |
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6 | 6 | In the original Berkeley Postgres system, there were several storage managers, |
7 | 7 | of which only the "magnetic disk" manager remains. (At Berkeley there were |
8 | 8 | also managers for the Sony WORM optical disk jukebox and persistent main |
9 | 9 | memory, but these were never supported in any externally released Postgres, |
10 | | -nor in any version of PostgreSQL.) However, we retain the notion of a storage |
11 | | -manager switch in case anyone wants to reintroduce other kinds of storage |
12 | | -managers. |
| 10 | +nor in any version of PostgreSQL.) The "magnetic disk" manager is itself |
| 11 | +seriously misnamed, because actually it supports any kind of device for |
| 12 | +which the operating system provides standard filesystem operations; which |
| 13 | +these days is pretty much everything of interest. However, we retain the |
| 14 | +notion of a storage manager switch in case anyone ever wants to reintroduce |
| 15 | +other kinds of storage managers. Removing the switch layer would save |
| 16 | +nothing noticeable anyway, since storage-access operations are surely far |
| 17 | +more expensive than one extra layer of C function calls. |
13 | 18 |
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14 | 19 | In Berkeley Postgres each relation was tagged with the ID of the storage |
15 | | -manager to use for it. This is gone. It would be more reasonable to |
16 | | -associate storage managers with tablespaces (a feature not present as this |
17 | | -text is being written, but one likely to emerge soon). |
| 20 | +manager to use for it. This is gone. It would beprobablymore reasonable |
| 21 | +toassociate storage managers with tablespaces, should we ever re-introduce |
| 22 | +multiple storage managers into the system catalogs. |
18 | 23 |
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19 | 24 | The files in this directory, and their contents, are |
20 | 25 |
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| 26 | + smgr.cThe storage manager switch dispatch code. The routines in |
| 27 | +this file call the appropriate storage manager to do storage |
| 28 | +accesses requested by higher-level code. smgr.c also manages |
| 29 | +the file handle cache (SMgrRelation table). |
| 30 | + |
| 31 | + md.cThe "magnetic disk" storage manager, which is really just |
| 32 | +an interface to the kernel's filesystem operations. |
| 33 | + |
21 | 34 | smgrtype.cStorage manager type -- maps string names to storage manager |
22 | 35 | IDs and provides simple comparison operators. This is the |
23 | | -regproc support for type'smgr' in the system catalogs. |
| 36 | +regproc support for type"smgr" in the system catalogs. |
24 | 37 | (This is vestigial since no columns of type smgr exist |
25 | 38 | in the catalogs anymore.) |
26 | 39 |
|
27 | | - smgr.cThe storage manager switch dispatch code. The routines in |
28 | | -this file call the appropriate storage manager to do hardware |
29 | | -accesses requested by the backend. smgr.c also manages the |
30 | | -file handle cache (SMgrRelation table). |
31 | | - |
32 | | - md.cThe magnetic disk storage manager. |
33 | | - |
34 | 40 | Note that md.c in turn relies on src/backend/storage/file/fd.c. |
35 | 41 |
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| 42 | + |
36 | 43 | Relation Forks |
37 | 44 | ============== |
38 | 45 |
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