"The goal of this patch series is to support reading and writing of a
"promoted index" - the Cassandra 2.* SSTable feature which allows reading
only a part of the partition without needing to read an entire partition
when it is very long. To make a long story short, a "promoted index" is
a sample of each partition's column names, written to the SSTable Index
file with that partition's entry. See a longer explanation of the index
file format, and the promoted index, here:
https://github.com/scylladb/scylla/wiki/SSTables-Index-File
There are two main features in this series - first enabling reading of
parts of partitions (using the promoted index stored in an sstable),
and then enable writing promoted indexes to new sstables. These two
features are broken up into smaller stand-alone pieces to facilitate the
review.
Three features are still missing from this series and are planned to be
developed later:
1. When we fail to parse a partition's promoted index, we silently fall back
to reading the entire partition. We should log (with rate limiting) and
count these errors, to help in debugging sstable problems.
2. The current code only uses the promoted index when looking for a single
contiguous clustering-key range. If the ck range is non-contiguous, we
fall back to reading the entire partition. We should use the promoted
index in that case too.
3. The current code only uses the promoted index when reading a single
partition, via sstable::read_row(). When scanning through all or a
range of partitions (read_rows() or read_range_rows()), we do not yet
use the promoted index; We read contiguously from data file (we do not
even read from the index file, so unsurprisingly we can't use it)."
(cherry picked from commit 700feda0db)
It was noticed that small sstables will accumulate for a column family because
scylla was limited to two compaction per shard, and a column family could have
at most one compaction running at a given shard. With the number of sstables
increasing rapidly, read performance is degraded.
At the moment, our compaction manager works by running two compaction task
handlers that run in parallel to the rest of the system. Each task handler
gets to run when needed, gets a column family from compaction manager queue,
runs compaction on it, and goes to sleep again. That's basically its cycle.
Compaction manager only allows one instance of a column family to be on its
queue, meaning that it's impossible for a column family to be compacted in
parallel. One compaction starts after another for a given column family.
To solve the problem described, we want to concurrently run compaction jobs
of a column family that have different "size tier" (or "weight").
For those unfamiliar, compaction job contains a list of sstables that will be
compacted together.
The "size tier" of a compaction job is the log of the total size of the input
sstables. So a compaction job only gets to run if its "size tier" is not the
same of an ongoing compaction. There is no point in compacting concurrently at
the same "size tier", because that slows down both compactions.
We will no longer queue column families in compaction manager. Instead, we
create a new fiber to run compaction on demand.
This fiber that runs asynchronously will do the following:
1) Get a compaction job from compaction strategy.
2) Calculate "size tier" of compaction job.
3) Run compaction job if its "size tier" is not the same of an ongoing
compaction for the given column family.
As before, it may decide to re-compact a column family based on a stat stored
in column family object.
Ran all compaction-related dtests.
Fixes#1216.
Reviewed-by: Nadav Har'El <nyh@scylladb.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <d30952ff136192a522bde4351926130addec8852.1462311908.git.raphaelsc@scylladb.com>
When we recreate the summary from a missing Summary, we should make
sure it is generated sanely, and that it resembles the Summary that
would have otherwise been there.
In this tests we'll grab one of the Summary tests we've been doing,
and just apply them to the non-existent Summary file. We expect
the same results on those cases. Plus, a new test is added with some
sanity checking.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
Vlad and I were working on finding the root of the problems with
refresh. We found that refresh was deleting existing sstable files
because of a bug in a function that was supposed to return the maximum
generation of a column family.
The intention of this function is to get generation from last element
of column_family::_sstables, which is of type std::map.
However, we were incorrectly using std::map::end() to get last element,
so garbage was being read instead of maximum generation.
If the garbage value is lower than the minimum generation of a column
family, then reshuffle_sstables() would set generation of all existing
sstables to a lower value. That would confuse our mechanism used to
delete sstables because sstables loaded at boot stage were touched.
Solution to this problem is about using rbegin() instead of end() to
get last element from column_family::_sstables.
The other problem is that refresh will only load generations that are
larger than or equal to X, so new sstables with lower generation will
not be loaded. Solution is about creating a set with generation of
live SSTables from all shards, and using this set to determine whether
a generation is new or not.
The last change was about providing an unused generation to reshuffle
procedure by adding one to the maximum generation. That's important to
prevent reshuffle from touching an existing SSTable.
Tested 'refresh' under the following scenarios:
1) Existing generations: 1, 2, 3, 4. New ones: 5, 6.
2) Existing generations: 3, 4, 5, 6. New ones: 1, 2.
3) Existing generations: 1, 2, 3, 4. New ones: 7, 8.
4) No existing generation. No new generation.
5) No existing generation. New ones: 1, 2.
I also had to adapt existing testcase for reshuffle procedure.
Fixes#1073.
Signed-off-by: Raphael Carvalho <raphaelsc@scylladb.com>
Message-Id: <1c7b8b7f94163d5cd00d90247598dd7d26442e70.1458694985.git.raphaelsc@scylladb.com>
Currently, if sstable::write_components() is called to write a new sstable
using the same generation of a sstable that exists, a temporary TOC will
be unconditionally created. Afterwards, the same sstable::write_components()
will fail when it reaches sstable::create_data(). The reason is obvious
because data component exists for that generation (in this scenario).
After that, user will not be able to boot scylla anymore because there is
a generation with both a TOC and a temporary TOC. We cannot simply remove a
generation with TOC and temporary TOC because user data will be lost (again,
in this scenario). After all, the temporary TOC was only created because
sstable::write_components() was wrongly called with the generation of a
sstable that exists.
Solution proposed by this patch is to trigger exception if a TOC file
exists for the generation used.
Some SSTable unit tests were also changed to guarantee that we don't try
to overwrite components of an existing sstable.
Refs #1014.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Message-Id: <caffc4e19cdcf25e4c6b9dd277d115422f8246c4.1457643565.git.raphaelsc@scylladb.com>
This patch makes sure that every time we need to create a new generation number -
the very first step in the creation of a new SSTable, the respective CF is already
initialized and populated. Failure to do so can lead to data being overwritten.
Extensive details about why this is important can be found
in Scylla's Github Issue #1014
Nothing should be writing to SSTables before we have the chance to populate the
existing SSTables and calculate what should the next generation number be.
However, if that happens, we want to protect against it in a way that does not
involve overwriting existing tables. This is one of the ways to do it: every
column family starts in an unwriteable state, and when it can finally be written
to, we mark it as writeable.
Note that this *cannot* be a part of add_column_family. That adds a column family
to a db in memory only, and if anybody is about to write to a CF, that was most
likely already called. We need to call this explicitly when we are sure we're ready
to issue disk operations safely.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
All the SSTable read path can now take an io_priority. The public functions will
take a default parameter which is Seastar's default priority.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
We have an API that wraps open_file_dma which we use in some places, but in
many other places we call the reactor version directly.
This patch changes the latter to match the former. It will have the added benefit
of allowing us to make easier changes to these interfaces if needed.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
Message-Id: <29296e4ec6f5e84361992028fe3f27adc569f139.1451950408.git.glauber@scylladb.com>
Since bytes is a very generic value that is returned from many calls,
it is easy to pass it by mistake to a function expecting a data_value,
and to get a wrong result. It is impossible for the data_value constructor
to know if the argument is a genuine bytes variable, a data_value of another
type, but serialized, or some other serialized data type.
To prevent misuse, make the data_value(bytes) constructor
(and complementary data_value(optional<bytes>) explicit.
We use boost::any to convert to and from database values (stored in
serlialized form) and native C++ values. boost::any captures information
about the data type (how to copy/move/delete etc.) and stores it inside
the boost::any instance. We later retrieve the real value using
boost::any_cast.
However, data_value (which has a boost::any member) already has type
information as a data_type instance. By teaching data_type intances about
the corresponding native type, we can elimiante the use of boost::any.
While boost::any is evil and eliminating it improves efficiency somewhat,
the real goal is growing native type support in data_type. We will use that
later to store native types in the cache, enabling O(log n) access to
collections, O(1) access to tuples, and more efficient large blob support.
The current codes assumes a particular dir/generation pair. We
will use it for a more generic case. This code could really use some
clean up, by the way. We should do it later.
Signed-off-by: Glauber Costa <glommer@scylladb.com>
There is no reason aside from testing for a table to just change its generation
number.
There will be, however, when we support loading new sstables. The method
however needs to be completely rewritten, so let's make sure the tests are not
using that.
Signed-off-by: Glauber Costa <glommer@scylladb.com>
Now that we are using the NSM, and not the general parser for the index, there
is no reason to keep using disk_string<>s in it. Since it is staying in the way
of further optimizations, let's get rid of it and use bytes directly.
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
read_indexes was one of the first functions coded in the sstable read path. At
the time, I made the (now so obviously) wrong decision to code it generic
enough so that we could specify the number of items to be read, instead of an
upper bound in the file.
The main reason for that, was that without the Summary, we have no way to know
where to stop reading, and the Summary is a relatively new addition to the C*
codebase: while I didn't really check when it got in, the code is full of tests
for its presence.
That turned out to be totally useless: we always read the indexes with the help
of the Summary. While the Summary is a relatively new addition to C*, it is
present in all version we aim to support. Meaning that reads without the
Summary will never happen in our codebase.
Even if, in the future, we happen to ditch the Summary file, we are very likely
to do so in favor of some other structure that also allows us to manipulate precise
borders in the Index.
The code as it is, however, would not be too big of a problem if that wasn't
causing us performance problems. But it is, and the majority of it is caused by
the fact that our underlying read_indexes do not know in advance how many bytes
to read, forcing us to do an element-per-element read.
It's time for a change.
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
When a schema is available, we use it. However, we have, by now, way too many
tests. Some of them use tables for which we don't even know the schema. It would
have been a massive amount of work to require a schema for all of them - so I am
keeping both constructors around.
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
