Split the `log_record` to `log_record_header` type that has the record
metadata fields and the mutation as a separate field which is the actual
record data:
struct log_record {
log_record_header header;
canonical_mutation mut;
};
Both the header and mutation have variable serialized size. When a
record is serialized in a write_buffer, we first put a small
`record_header` that has the header size and data size, then the
serialized header and data follow. The `log_location` of a record points
to the beginning of the `record_header`, and the size includes the
`record_header`.
This allows us to read a record header without reading the data when
it's not needed and avoid deserializing it:
* on recovery, when scanning all segments, we read only the record
headers.
* on compaction, we read the record header first to determine if the
record is alive, if yes then we read the data.
Closesscylladb/scylladb#29457
Three bugs fixed in segment_manager.cc:
1. write_to_separator(): captured [&index] where index was a local
coroutine-frame reference. The future is stored in
buf.pending_updates and resolved later in flush_separator_buffer(),
by which time the enclosing coroutine frame is destroyed, making
&index a dangling pointer. This is a use-after-free that manifests
as a segfault. Fix: capture index_ptr (raw pointer by value) instead.
2. add_segment_to_compaction_group(): same dangling [&index] pattern
inside the for_each_live_record lambda during recovery. Same fix
applied.
3. write(): local 'auto loc = seg->allocate(...)' shadowed the outer
'log_location loc', causing the function to always return a
zero-initialized log_location{}. Fix: remove 'auto' so the
assignment targets the outer variable.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Closesscylladb/scylladb#29451
add the function table::take_logstor_snapshot that is similar to
take_storage_snapshot for sstables.
given a token range, for each storage group in the range, it flushes the
separator buffers and then makes a snapshot of all segments in the sg's
compaction groups while disabling compaction.
the segment snapshot holds a reference to the segment so that it won't
be freed by compaction, and it provides an input stream for reading the
segment.
this will be used for tablet migration to stream the segments.
add functions for creating segment input and output streams, that will
be used for segment streaming.
the segment input stream creates a file input stream that reads a given
segment.
the segment output stream allocates a new local segment and creates an
output stream that writes to the segment, and when closed it loads the
segment and adds it to the compaction group.
implement compaction group cleanup by clearing the range in the index
and discarding the segments of the compaction group.
segments are discarded by overwriting the segment header to indicate the
segment is empty while preserving the segment generation number in order
to not resurrect old data in the segment.
implement tablet split for logstor.
flush the separator and then perform split as a new type of compaction:
take a batch of segments from the source compaction group, read them and
write all live records into left/right write buffers according to the
split classifier, flush them to the compaction group, and free the old
segments. segments that fit in a single target compaction group are
removed from the source and added to the correct target group.
implement tablet merge with logstor.
disable compaction for the new compaction group, then merge the merging
compaction groups by merging their logstor segments set into the new cg
- simply merging the segment histogram.
add a function that stops and disabled compaction for a compaction group
and returns a compaction reenabler object, similarly to the normal
compaction manager.
this will be useful for disabling compaction while doing operations on
the compaction group's logstor segment set.
we have two types of segments. the active segment is "mixed" because we
can write to it multiple write_buffers, each write buffer having records
from different tables and tablets. in constrast, the separator and
compaction write "full" segments - they write a single write_buffer that
has records from a single tablet and storage group.
for "full" segments, we add a segment header the contains additional
useful metadata such as the table and token range in the segment.
the write buffer header contains the type of the buffer, mixed or full.
if it's full then it has a segment header placed after the write buffer
header.
previously when writing to the active segment, the allocation was
serialized but multiple writes could proceed concurrently to different
offsets. change it instead to serialize the entire write.
we prefer to write larger buffers sequentially instead of multiple
buffers concurrently. it is also better that we don't have "holes" in
the segment.
we also change the buffered_writer to send a single flushing buffer at a
time. it has a ring of buffers, new writes are written to the head
buffer, and a single consumer flushes the tail buffer.
extend compaction_group functions such as disk size calculation and
empty() to account also for the logstor segments that the compaction
group owns.
reuse the sstable_add_gate when there is a write in process to a
compaction group, in order for the compaction group to be considered not
empty.
add the function table::compaction_group_for_logstor_segment that we use
when recovering a segment to find the compaction group for a segment
based on its token range, similarly to compaction_group_for_sstable for
sstables.
extract the common logic from compaction_group_for_sstable to a common
function compaction_group_for_token_range that finds a compaction group
for a token range.
The version of fmt installed on my machine refuses to work with
`std::filesystem::path` directly. Add `.string()` calls in places that
attempt to print paths directly in order to make them work.
Closesscylladb/scylladb#29148
A compaction group has a separator buffer that holds the mixed segments
alive until the separator buffer is flushed. A mixed segment can be
freed only after all separator buffers that hold writes from the segment
are flushed.
Typically a separator buffer is flushed when it becomes full. However
it's possible for example that one compaction groups is filled slower
than others and holds many segments.
To fix this we trigger a separator flush periodically for separator
buffers that hold old segments. We track the active segment sequence
number and for each separator buffer the oldest sequence number it
holds.
Fix the logstor recovery to work with compaction groups. When recovering
a segment find its token range and add it to the appropriate compaction
groups. if it doesn't fit in a single compaction group then write each
record to its compaction group's separator buffer.
Change the primary index to be a btree that is ordered by token,
similarly to a memtable, and create a index per-table instead of a
single global index.
Add a segment_set member to replica::compaction_group that manages the
logstor segments that belong to the compaction group, similarly to how
it manages sstables. Add also a separator buffer in each compaction
group.
When writing a mutation to a compaction group, the mutation is written
to the active segment and to the separator buffer of the compaction
group, and when the separator buffer is flushed the segment is added to
the compaction_group's segment set.
add a write gate to write_buffer. when writing a record to the write
buffer, the gate is held and passed back to the caller, and the caller
holds the gate until the write operation is complete, including
follow-up operations such as updating the index after the write.
in particular, when writing a mutation in logstor::write, the write
buffer is held open until the write is completed and updated in the
index.
when writing the write buffer to the active segment, we write the buffer
and then wait for the write buffer gate to close, i.e. we wait for all
index updates to complete before proceeding. the segment is held open
until all the write operations and index updates are complete.
this property is useful for correctness: when a segment is closed we
know that all the writes to it are updated in the index. this is needed
in compaction for example, where we take closed segments and check
which records in them are alive by looking them up in the index. if the
index is not updated yet then it will be wrong.
implement freeing all segments of a table for table truncate.
first do barrier to flush all active and mixed segments and put all the
table's data in compaction groups, then stop compaction for the table,
then free the table's segments and remove the live entries from the
index.
add barrier operation that forces switch of the active segment and
separator, and waits for all existing segments to close and all
separators to flush.
add tracking of the total separator debt - writes that were written to a
separator and waiting to be flushed, and add flow control to keep the
debt in control by delaying normal writes.
on recovery we may find mixed segments. recover them by adding them to a
separator, reading all their records and writing them to the separator,
and flush the separator.
we free a segment from compaction after updating all live records in the
segment to point to new locations in the index. we need to ensure they
are no running operations that use the old locations before we free the
segment.
initial implementation of the separator. it replaces "mixed" segments -
segments that have records from different groups, to segments by group.
every write is written to the active segment and to a buffer in the
active separator. the active separator has in-memory buffers by group.
at some threshold number of segments we switch the active segment and
separator atomically, and start flushing the separator.
the separator is flushed by writing the buffers into new non-mixed
segments, adding them to a compaction group, and frees the mixed
segments.
initial and basic recovery implementation.
* find all files, read their segments and populate the index with the
newest record for each key.
* find which segments are used and build the usage histogram
add segment generation number that is incremented when the segment is
reused, and it's written to every buffer that is written to the segment.
this is useful for recovery.
reserve segments for compaction so it always has enough segments to run
and doesn't get stuck.
do the compaction writes into full new segments instead of the active
segment.
add group_id value to each log record that is passed with the mutation
when writing it.
the group_id will be used to group log records in segments, such that a
segment will contain records only from a single group.
this will be useful for tablet migration. we want for each tablet to
have their own segments with all their records, so we can migrate them
efficiently by copying these segments.
the group_id value is set to a value equivalent to the tablet id.
basic utility for generation numbers that will be useful next. a
generation number is an unsigned integer that can be incremented and
compared even if it wraparounds, assuming the values we compare were
written around the same time.
initial implementation of the logstor storage engine for key-value
tables that supports writes, reads and basic compaction.
main components:
* logstor: this is the main interface to users that supports writing and
reading back mutations, and manages the internal components.
* index: the primary index in-memory that maps a key to a location on
disk.
* write buffer: writes go initially to a write buffer. it accumulates
multiple records in a buffer and writes them to the segment manager in
4k sized blocks.
* segment manager: manages the storage - files, segments, compaction. it
manages file and segment allocation, and writes 4k aligned buffers to
the active segment sequentially. it tracks the used space in each
segment. the compaction finds segment with low space usage and writes
them to new segments, and frees the old segments.
