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/*! @m_page{{c,java},lsm,Log-Structured Merge Trees}
@section lsm_background Background
A common requirement is sustained throughput under a workload that consists
of random inserts, where either the key range is chosen so that inserts are
very unlikely to conflict (e.g., 128-bit hashes), or where inserts are
expected to overwrite existing values.
With traditional btree variants, inserts are very fast while the data set
remains in cache, but once the tree overflows the cache, performance drops
significantly. There are two factors involved:
-# once the data fills the cache, new inserts have some probability of going
to a page that is not in cache, requiring a read; and
-# the cache is full of dirty pages, so pages have to be written to free
space in the cache before the read can be satisfied.
@section lsm_description Description of LSM trees
Log-Structured Merge Trees are described in this paper by Patrick O'Neil,
Edward Cheng, Dieter Gawlick and Elizabeth O'Neil:
http://www.cs.umb.edu/~poneil/lsmtree.pdf
A logical tree is split into several physical pieces so that the
most-recently-updated portion of data is in a tree that fits entirely in
memory. The size of the in-memory chunk can be configured with the
\c "lsm=(chunk_size)" configuration key to WT_SESSION::create.
Once the in-memory tree reaches a threshold size, a new in-memory tree is
created and the old tree synced to disk. Once written to disk, trees are
read-only, though they are merged in the background with other on-disk
trees to reduce the cost of reads.
With this structure, "blind writes" can be performed entirely on the
in-memory tree. Deletes are implemented by inserting a special "tombstone"
record into the in-memory tree.
@section lsm_api Interface to LSM trees
An LSM tree can be created as follows, in much the same way as a
WiredTiger btree file:
@code
@m_if{c}
session->create(session, "table:bucket", "type=lsm,key_format=S,value_format=S");
@m_else
session.create("table:bucket", "type=lsm,key_format=S,value_format=S");
@m_endif
@endcode
Once created, the LSM tree can be accessed using the same cursor interface
as other data sources in WiredTiger:
@code
@m_if{c}
WT_CURSOR *c;
session->open_cursor(session, "table:bucket", NULL, NULL, &c);
for(;;) {
c->set_key(c, "key");
c->set_value(c, "value");
c->insert(c);
}
@m_else
Cursor c = session.open_cursor("table:bucket", null, null);
for(;;) {
c.putKeyString("key");
c.putValueString("value");
c.insert();
}
@m_endif
@endcode
If an LSM cursor is configured with \c "overwrite=false" passed to
WT_SESSION::open_cursor, the result will be a search through the levels of the
LSM tree before every modification.
@section lsm_merge Merging
A background thread is opened for each active LSM tree. This thread is
responsible for both writing old chunks to stable storage, and for merging
multiple chunks together so that reads can be satisfied from a small number
of files. There is currently no way to configure merges: they are performed
automatically by the background thread.
@section lsm_bloom Bloom filters
WiredTiger creates a Bloom filter when merging. This is an additional file
that contains a configurable number of bits per key (default 8). Keys are
hashed a configurable number of times (default 4), and the corresponding
bits set. The Bloom filter is used to avoid reading from a chunk if the key
cannot be present.
With the defaults, the Bloom filter only requires one byte per key, so
it usually fits in cache. The Bloom parameters can be configured with
\c "lsm=(bloom_bit_count)" and \c "lsm=(bloom_hash_count)" configuration
keys to WT_SESSION::create. The Bloom file can be configured with the
\c "lsm=(bloom_config)" key.
@section lsm_schema Creating tables using LSM trees
Tables or indices can be stored using LSM trees. Schema support is provided
for LSM as an extension to the WT_SESSION::create method:
@code
@m_if{c}
session->create(session, "table:T", "type=lsm");
@m_else
session.create("table:T", "type=lsm");
@m_endif
@endcode
The default type for all schema objects will continue to be btree.
@section lsm_caveats Caveats
@subsection lsm_hazard Hazard configuration
Reads from an LSM cursor may need to position a cursor in each active chunk.
The number of chunks depends on the chunk size, and how many chunks have
been merged. There must be at least as many hazard pointers available as
there are chunks in the tree for each cursor that is open on the LSM tree.
The number of hazard pointers is configured with the \c "hazard_max"
configuration key to ::wiredtiger_open.
@subsection lsm_checkpoints Named checkpoints
Named checkpoints are not supported on LSM trees, and cursors cannot be
opened with a non-empty \c "checkpoint" configuration (that is, only the
most recent standard checkpoint can be read).
*/
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