This adds write path support for segmented chunk data files.
Files of 512MB are pre-allocated and written to. If the file size
is exceeded, the next file is started. On completion, files
are truncated to their final size.
This is an initial (and hacky) first pass on allowing
appending to multiple blocks simultaniously to avoid
dropping samples right after cutting a new head block.
It's also required for cases like the PGW, where a scrape may
contain varying timestamps.
The former approach created unordered postings list by either
map iteration of new series being unsorted (fixable) or concurrent
writers creating new series interleaved.
We switch back to generating ephemeral references for a single batch.
Newly created series have to be re-set upon the next insert.
This exposes a reference number of a series represented by a label set
to clients.
Subsequent samples can be directly added via the reference rather than
repeatedly passing in the full labels. This drasitcally speeds up the
append process.
The appender chain uses different sections of the reference number for
assignment to child appenders and invalidating reference numbers as
necessary.
Clients can either pass out reference numbers themselves or have their
own optimized lookup, i.e. by directly associating unparsed metric
descriptors strings with reference numbers.
This adds a memory series holding several chunk to replace
the single head chunk per series so far.
This is necessary for uniform maximum chunk sizes in cases
where some series have higher frequency samples than others.
This adds a 4 sample buffer to every head chunk. The XOR
compression scheme may edit bytes in place. The minimum size
of a sample is 2 bits. So keeping the last 4 samples in an in-memory
buffer makes it safe to query the preceeding ones while samples
are added
This adds a position mapper that takes series from a head block
in the order they were appended and creates a mapping representing
them in order of their label sets.
Write-repair of the postings list would cause very expensive writing.
Hence, we keep them as they are and only apply the postition mapping
at the very end, after a postings list has been sufficienctly reduced
through intersections etc.
This initializes the chunkDesc's last timestamp to the minimum
value so initial samples with a timestamp of 0 (e.g. in tests)
are not accidentally dropped.
