# TimeWindow Compaction Strategy, TTL, Tombstones, and window size **URL:** \ **Category:** ScyllaDB\ **Tags:** data-model, ttl, compaction, tombstone\ **Created:** [April 15, 2024, 6:31pm UTC](https://forum.scylladb.com/t/timewindow-compaction-strategy-ttl-tombstones-and-window-size/1485 "2024-04-15T18:31:07Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![Guy](https://sea2.discourse-cdn.com/flex016/user_avatar/forum.scylladb.com/guy/32/9_2.png) [@Guy](https://forum.scylladb.com/u/Guy)\ **Post date:** [April 15, 2024, 6:31pm UTC](https://forum.scylladb.com/t/timewindow-compaction-strategy-ttl-tombstones-and-window-size/1485/1 "2024-04-15T18:31:07Z")
> [@](#): > > [**Originally from the User Slack**](https://scylladb-users.slack.com/) > > ![Daria_Fedorova](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/d64bdc4c9feed87b06be17719f3ebe4ca891bdba.jpeg) **@Daria\_Fedorova:** Hi, I have a question about Time Window CompactionStrategy > I need to add TTL but I can’t just use 1 TTL value for all records (2 values maybe and a small number of records without TTL ) And I understand that it is not ideal if I leave them all in one table, but shouldn’t tombstone compaction solve the problem of memory release ? > This will add strain on disk probably but are there any other problems with multiple TTL in one table ? My only other alternative is joining the the time-series from 3 table on client side (thankfully it is not so difficult) > > ![Felipe_Cardeneti_Mendes](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/233a9d524fbc97e39f08633c16b3dd2575d1d1f4.jpeg) **@Felipe\_Cardeneti\_Mendes:** you need to remember that a bucket will only be deleted after all records expire. If you are using 2 different TTLs, then you probably should select a window size that matches your largest TTL. > For records with no TTL you shouldn’t be using TWCS, use another strategy instead > > ![Daria_Fedorova](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/d64bdc4c9feed87b06be17719f3ebe4ca891bdba.jpeg) **@Daria\_Fedorova:** I did not find anything on how to choose window size for TWCS , the one we have I think is a compromise for fast reads for a couple of days , it also maybe was chosen when we had different disks - this memory is lost , maybe 1 week is too small window > > cassandra doc says something about it but they don’t explain the reasons > > ```auto > Ideally, operators should select a compaction_window_unit and compaction_window_size pair that produces approximately 20-30 windows - if writing with a 90 day TTL, for example, a 3 Day window would be a reasonable choice ('compaction_window_unit':'DAYS','compaction_window_size':3). > > ``` > > The larger ttl will be 3 or 5 years , sstables will grow very big , is it not a problem ? > > if I follow cassandra doc then window size for TTL = 3 years is around 36 days > > Can you explain what factors to consider when choosing a window ? > i think that reads are faster if they work with 1 sstable and for this window == 1 week seems reasonable for us > > What is the motivation to make it bigger ? > > ![Felipe_Cardeneti_Mendes](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/233a9d524fbc97e39f08633c16b3dd2575d1d1f4.jpeg) **@Felipe\_Cardeneti\_Mendes:** You need to remember that SSTables will only be evicted after all data within them expires (hence why you shouldn’t use TWCS if you have records with no TTL). > > If you have a window of 1 week, but 3 years of TTL, then that will result in \>150 windows, which may create heavy memory pressure and slow down your reads/repairs over time. > > You want to find a good balance between your reads and fewer windows to avoid having too many open files to satisfy a read. > ie: consider a 1 hour window and 30 day TTL. Reading all records will require scanning through 720 windows. 💣 > > ![Daria_Fedorova](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/d64bdc4c9feed87b06be17719f3ebe4ca891bdba.jpeg) **@Daria\_Fedorova:** Ok I see ) > that is not a big concern for us > mostly reads requests a couple of days or month > and we split very long reads on client (api) anyway > > ![Felipe_Cardeneti_Mendes](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/233a9d524fbc97e39f08633c16b3dd2575d1d1f4.jpeg) **@Felipe\_Cardeneti\_Mendes:** a last thing is that you shouldn’t generally worry much about large SSTables, TWCS compacts everything altogether on a per-window basis to optimize for reads. Plus SSTables are further split by shard. > > ![Daria_Fedorova](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/d64bdc4c9feed87b06be17719f3ebe4ca891bdba.jpeg) **@Daria\_Fedorova:** I ask a college and he said that they did not want a bigger window because last window uses Size Tiered Compaction and with bigger window when we read range of last records for user it will iterate over many sstables ? > > and another concern was longer compact ( it affects latency a little now , idk if it will get worse ) > > ![Felipe_Cardeneti_Mendes](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/233a9d524fbc97e39f08633c16b3dd2575d1d1f4.jpeg) **@Felipe\_Cardeneti\_Mendes:** all windows use STCS > > ![Daria_Fedorova](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/d64bdc4c9feed87b06be17719f3ebe4ca891bdba.jpeg) **@Daria\_Fedorova:** thx > > ![Felipe_Cardeneti_Mendes](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/233a9d524fbc97e39f08633c16b3dd2575d1d1f4.jpeg) **@Felipe\_Cardeneti\_Mendes:** by the way, we are going to have a compaction talk in our upcoming ScyllaDB Summit [scylladb.com/summit](http://scylladb.com/summit) 😉 > > ![Daria_Fedorova](https://us1.discourse-cdn.com/flex016/uploads/scylladb/original/1X/d64bdc4c9feed87b06be17719f3ebe4ca891bdba.jpeg) **@Daria\_Fedorova:** yes, I am planning to tune in