I know early removal of tombstones is dangerous because it can cause deleted data to be resurrected, but if all replicas have confirmed deletion then such removal should be safe. For example, if a table has replication factor 3 and all 3 nodes containing the key have confirmed that they have the appropriate tombstone, it should be safe to perform a compaction in which the tombstones are removed because there would be no lingering copies of the data.
Is such safe removal of tombstones possible in Cassandra?
I would rather set gc_grace_seconds to infinity and rely on this type of safe compaction of tombstones than worry about the timing of nodetool repair and gc_grace_seconds.
No, it's not possible to remove tombstones without changing your gc_grace_seconds.
There are operations where all 3 replicas may ack the tombstone, remove it, then need it later on. Consider the case where you need to stream an earlier SSTable back into a cluster.
This type of manual tombstone removal will be significantly worse performance wise since you'll only be doing it periodically rather than constantly. You'll be reading extra data unnecessarily as well as constantly compacting tombstones that should be removed.
My recommendation is to set your gc_grace_seconds to something reasonable (10 days is fine) and schedule repairs using opscenter or cassandra-reaper.
Related
We are using Cassandra 3.10 with 6 nodes cluster.
lately, we noticed that our data volume increased drastically, approximately 4GB per day in each node.
We want to implement a more aggressive retention policy in which we will change the compaction to TWCS with 1-hour window size and set a few days TTL, this can be achieved via the table properties.
Since the ETL should be a slow process in order to lighten Cassandra workload it possible that it will not finish extracting all the data until the TTL, so I wanted to know is there a way for the ETL process to set TTL=0 on entire SSTable once it done extracting it?
TTL=0 is read as a tombstone. When next compacted it would be written tombstone or purged depending on your gc_grace. Other than the overhead of doing the writes of the tombstone it might be easier just to do a delete or create sstables that contain the necessary tombstones than to rewrite all the existing sstables. If its more efficient to do range or point tombstones will depend on your version and schema.
An option that might be easiest is to actually use a different compaction strategy all together or a custom one like https://github.com/protectwise/cassandra-util/tree/master/deleting-compaction-strategy. You can then just purge data on compactions that have been processed. This still depends quite a bit on your schema on how hard it would be to mark whats been processed or not.
You should set TTL 0 on table and query level as well. Once TTL expire data will converted to tombstones. Based on gc_grace_seconds value next compaction will clear all the tombstones. you may run major compaction also to clear tombstones but it is not recommended in cassandra based on compaction strategy. if STCS atleast 50% disk required to run healthy compaction.
I'm using LCS and a relatively large TTL of 2 years for all inserted rows and I'm concerned about the moment at which C* would drop the corresponding tombstones (neither explicit deletes nor updates are being performed).
From Missing Manual for Leveled Compaction Strategy, Tombstone Compactions in Cassandra and Deletes Without Tombstones or TTLs I understand that
All levels except L0 contain non-overlapping SSTables, but a partition key may be present in one SSTable in each level (aka distributed in all levels).
For a compaction to be able to drop a tombstone it must be sure that is compacting all SStables that contains de data to prevent zombie data (this is done checking bloom filters). It also considers gc_grace_seconds
So, for my particular use case (2 years TTL and write heavy load) I can conclude that TTLed data will be in highest levels so I'm wondering when those SSTables with TTLed data will be compacted with the SSTables that contains the corresponding SSTables.
The main question will be: Where are tombstones (from ttls) being created? Are being created at Level 0 so it will take a long time until it will end up in the highest levels (hence disk space will take long time to be freed)?
In a comment from About deletes and tombstones Alain says that
Yet using TTLs helps, it reduces the chances of having data being fragmented between SSTables that will not be compacted together any time soon. Using any compaction strategy, if the delete comes relatively late in the row history, as it use to happen, the 'upsert'/'insert' of the tombstone will go to a new SSTable. It might take time for this tombstone to get to the right compaction "bucket" (with the rest of the row) and for Cassandra to be able to finally free space.
My understanding is that with TTLs the tombstones is created in-place, thus it is often and for many reasons easier and safer to get rid of a TTLs than from a delete.
Another clue to explore would be to use the TTL as a default value if that's a good fit. TTLs set at the table level with 'default_time_to_live' should not generate any tombstone at all in C*3.0+. Not tested on my hand, but I read about this.
I'm not sure what it means with "in-place" since SSTables are immutable.
(I also have some doubts about what it says of using default_time_to_live that I've asked in How default_time_to_live would delete rows without tombstones in Cassandra?).
My guess is that is referring to tombstones being created in the same level (but different SStables) that the TTLed data during a compaction triggered by one of the following reasons:
"Going from highest level, any level having score higher than 1.001 can be picked by a compaction thread" The Missing Manual for Leveled Compaction Strategy
"If we go 25 rounds without compacting in the highest level, we start bringing in sstables from that level into lower level compactions" The Missing Manual for Leveled Compaction Strategy
"When there are no other compactions to do, we trigger a single-sstable compaction if there is more than X% droppable tombstones in the sstable." CASSANDRA-7019
Since tombstones are created during compaction, I think it may be using SSTable metadata to estimate droppable tombstones.
So, compactions (2) and (3) should be creating/dropping tombstones in highest levels hence using LCS with a large TTL should not be an issue per se.
With creating/dropping I mean that the same kind of compactions will be creating tombstones for expired data and/or dropping tombstones if the gc period has already passed.
A link to source code that clarifies this situation will be great, thanks.
Alain Rodriguez's answer from mailing list
Another clue to explore would be to use the TTL as a default value if
that's a good fit. TTLs set at the table level with 'default_time_to_live'
should not generate any tombstone at all in C*3.0+. Not tested on my hand,
but I read about this.
As explained on a parallel thread, this is wrong, mea culpa. I believe the rest of my comment still stands (hopefully :)).
I'm not sure what it means with "in-place" since SSTables are immutable.
My guess is that is referring to tombstones being created in the same
Yes, I believe during the next compaction following the expiration date,
the entry is 'transformed' into a tombstone, and lives in the SSTable that
is the result of the compaction, on the level/bucket this SSTable is put
into. That's why I said 'in-place' which is indeed a bit weird for
immutable data.
As a side idea for your problem, on 'modern' versions of Cassandra (I don't
remember the version, that's what 'modern' means ;-)), you can run
'nodetool garbagecollect' regularly (not necessarily frequently) during the
off-peak period. That might use the cluster resources when you don't need
them to claim some disk space. Also making sure that a 2 years old record
is not being updated regularly by design would definitely help. In the
extreme case of writing a data once (never updated) and with a TTL for
example, I see no reason for a 2 years old data not to be evicted
correctly. As long as the disk can grow, it should be fine.
I would not be too much scared about it, as there is 'always' a way to
remove tombstones. Yet it's good to think about the design beforehand
indeed, generally, it's good if you can rotate the partitions over time,
not to reuse old partitions for example.
I have recently lowered gc_grace_seconds for a CQL table. I am running LeveledCompactionStrategy. Is it possible for me to force purging of old tombstones from my SSTables?
TL;DR
Your tombstones will disappear on their own through compaction bit make sure you are running repair or they may come back from the dead.
http://www.datastax.com/documentation/cassandra/2.0/cassandra/dml/dml_about_deletes_c.html
Adding some more details:
Tombstones are not immediately available for deletion until both:
1) gc_grace_seconds has expired
2) they meet the requirements configured in tombstone compaction sub-properties
I need to free up disk by expiring tombstones, how do I do this quickly?
1) Run a repair to ensure your tombstones are consistent
2) Decrease gc_grace_seconds for your table (alter table statement)
3) Configure your compaction sub properties to speed up tombstone removal:
Decrease tombstone_compaction_interval and decrease tombstone_threshold, or set unchecked_tombstone_compaction to true to ignore both conditions and collect based purely on gc grace.
Is it working?
You can see statistics about tombstones in nodetool cfstats and by using the sstable metatdata utility found in your tools directory sstablemetadata <sstable filenames>.
In Cassandra 2.x when I delete one or multiple columns, they receive a tombstone in the Memtable but the data is not removed. At some point, the Memtable is flushed to an SSTable including the deleted data and the tombstone. When compaction is running, it will retain the tombstone with the specified grace period. What happens to the data? I have deleted a bunch of columns last week - less than gc_grace_seconds ago. I am not sure compaction has run yet. I haven't seen any change on disk size used yet, so I was wondering at which point is the data physically removed from disk?
In Cassandra 2.x when I delete one or multiple columns, they receive a tombstone in the Memtable but the data is not removed. At some point, the Memtable is flushed to an SSTable including the deleted data and the tombstone. When compaction is running, it will retain the tombstone with the specified grace period.
True.
What happens to the data?
The data will remain on disk at least for gc_grace_seconds. Next minor compaction right after gc_grace_seconds may remove it, but real timing depends mostly on your dataset and workload type.
I have deleted a bunch of columns last week - less than gc_grace_seconds ago. I am not sure compaction has run yet. I haven't seen any change on disk size used yet, so I was wondering at which point is the data physically removed from disk?
If you want to free some disk space, you can:
wait for gc_grace_seconds for normal minor compaction.
run nodetool compact which will trigger major compaction on current node freeing disk space right now.
My question is very simple. Is it in any way possible to retrieve columns that have been marked tombstone before the GCGraceSeconds period expiry(default 10 days). If yes what would be the exact CQL query for that?
If I were to understand the deletion process the tombstones are marked on the MemTables and the SSTable being immutable waiting for compaction still has the the deleted data waiting for compaction. So before compaction occurs is there any way to read the tombstoned data from either the Memtable or SSTable?
Using CQL 3.0 on CQLSH command prompt & Cassandra 2.0.
You are right, when a tombstone is inserted it usually doesn't immediately delete the underlying data (unless all your data is in a memtable). However, you can't control when it does. If you don't have much data and compaction happens quickly, the underlying data may be deleted very quickly, much sooner than 10 days.
There is no query to read deleted data, but you can inspect all your SSTables with sstable2json to see if they contain the deleted data.
Just to add on to the previous comment. Have a low value of gc_grace_seconds for the column families that have frequent deletions. It will take some time for gc but tombstones are expected to get cleared .