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mdbx-doc: describing the caveat of large values & huge transactions with a huge number of retired pages.

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Resolves https://github.com/erthink/libmdbx/issues/177

Change-Id: Id5f643f2ee78894293f73c3ad1789dc0a99ecffb
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Leonid Yuriev
2021-05-05 01:29:21 +03:00
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docs/_restrictions.md
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@@ -162,6 +162,34 @@ The "next" version of libmdbx (\ref MithrilDB) will completely solve this.
not apply to write transactions if the system clears stale writers, see
above.
## Large data items and huge transactions
MDBX allows you to store values up to 1 gigabyte in size, but this is
not the main functionality for a key-value storage, but an additional
feature that should not be abused. Such long values are stored in
consecutive/adjacent DB pages, which has both pros and cons. This allows
you to read long values directly without copying and without any
overhead from a linear section of memory.
On the other hand, when putting such values in the database, it is
required to find a sufficient number of free consecutive/adjacent
database pages, which can be very difficult and expensive, moreover
sometimes impossible since b-tree tends to fragmentation. So, when
placing very long values, the engine may need to process the entire GC,
and in the absence of a sufficient sequence of free pages, increase the
DB file. Thus, for long values, MDBX provides maximum read performance
at the expense of write performance.
A similar situation can be with huge transactions, in which a lot of
database pages are retired. The retired pages should be put into GC as a
list of page numbers for future reuse. But in huge transactions, such a
list of retired page numbers can also be huge, i.e. it is a very long
value and requires a long sequence of free pages to be saved. Thus, if
you delete large amounts of information from the database in a single
transaction, MDBX may need to increase the database file to save the
list of pages to be retired.
Both of these issues will be addressed in MithrilDB.
## Space reservation
An MDBX database configuration will often reserve considerable unused