Hello Akhil, thanks for your great blog post. One thing I cannot bring together: In the answer mail you write: "Note the cleanup threshold is .50 of 1GB and not a combination of heap and off heap space." In your blog post you write: "memtable_cleanup_threshold is the default value i.e. 33 percent of the total memtable heap and off heap memory."
Could you clarify this? Thanks Stefan 2017-05-30 2:43 GMT+02:00 Akhil Mehra <akhilme...@gmail.com>: > Hi Preetika, > > After thinking about your scenario I believe your small SSTable size might > be due to data compression. By default, all tables enable SSTable > compression. > > Let go through your scenario. Let's say you have allocated 4GB to your > Cassandra node. Your *memtable_heap_space_in_mb* and > > *memtable_offheap_space_in_mb *will roughly come to around 1GB. Since > you have memtable_cleanup_threshold to .50 table cleanup will be > triggered when total allocated memtable space exceeds 1/2GB. Note the > cleanup threshold is .50 of 1GB and not a combination of heap and off heap > space. This memtable allocation size is the total amount available for all > tables on your node. This includes all system related keyspaces. The > cleanup process will write the largest memtable to disk. > > For your case, I am assuming that you are on a *single node with only one > table with insert activity*. I do not think the commit log will trigger a > flush in this circumstance as by default the commit log has 8192 MB of > space unless the commit log is placed on a very small disk. > > I am assuming your table on disk is smaller than 500MB because of > compression. You can disable compression on your table and see if this > helps get the desired size. > > I have written up a blog post explaining memtable flushing ( > http://abiasforaction.net/apache-cassandra-memtable-flush/) > > Let me know if you have any other question. > > I hope this helps. > > Regards, > Akhil Mehra > > > On Fri, May 26, 2017 at 6:58 AM, preetika tyagi <preetikaty...@gmail.com> > wrote: > >> I agree that for such a small data, Cassandra is obviously not needed. >> However, this is purely an experimental setup by using which I'm trying to >> understand how and exactly when memtable flush is triggered. As I mentioned >> in my post, I read the documentation and tweaked the parameters accordingly >> so that I never hit memtable flush but it is still doing that. As far the >> the setup is concerned, I'm just using 1 node and running Cassandra using >> "cassandra -R" option and then running some queries to insert some dummy >> data. >> >> I use the schema from CASSANDRA_HOME/tools/cqlstress-insanity-example.yaml >> and add "durable_writes=false" in the keyspace_definition. >> >> @Daemeon - The previous post lead to this post but since I was unaware of >> memtable flush and I assumed memtable flush wasn't happening, the previous >> post was about something else (throughput/latency etc.). This post is >> explicitly about exactly when memtable is being dumped to the disk. Didn't >> want to confuse two different goals that's why posted a new one. >> >> On Thu, May 25, 2017 at 10:38 AM, Avi Kivity <a...@scylladb.com> wrote: >> >>> It doesn't have to fit in memory. If your key distribution has strong >>> temporal locality, then a larger memtable that can coalesce overwrites >>> greatly reduces the disk I/O load for the memtable flush and subsequent >>> compactions. Of course, I have no idea if the is what the OP had in mind. >>> >>> >>> On 05/25/2017 07:14 PM, Jonathan Haddad wrote: >>> >>> Sorry for the confusion. That was for the OP. I wrote it quickly right >>> after waking up. >>> >>> What I'm asking is why does the OP want to keep his data in the memtable >>> exclusively? If the goal is to "make reads fast", then just turn on row >>> caching. >>> >>> If there's so little data that it fits in memory (300MB), and there >>> aren't going to be any writes past the initial small dataset, why use >>> Cassandra? It sounds like the wrong tool for this job. Sounds like >>> something that could easily be stored in S3 and loaded in memory when the >>> app is fired up. >>> >>> On Thu, May 25, 2017 at 8:06 AM Avi Kivity <a...@scylladb.com> wrote: >>> >>>> Not sure whether you're asking me or the original poster, but the more >>>> times data gets overwritten in a memtable, the less it has to be compacted >>>> later on (and even without overwrites, larger memtables result in less >>>> compaction). >>>> >>>> On 05/25/2017 05:59 PM, Jonathan Haddad wrote: >>>> >>>> Why do you think keeping your data in the memtable is a what you need >>>> to do? >>>> On Thu, May 25, 2017 at 7:16 AM Avi Kivity <a...@scylladb.com> wrote: >>>> >>>>> Then it doesn't have to (it still may, for other reasons). >>>>> >>>>> On 05/25/2017 05:11 PM, preetika tyagi wrote: >>>>> >>>>> What if the commit log is disabled? >>>>> >>>>> On May 25, 2017 4:31 AM, "Avi Kivity" <a...@scylladb.com> wrote: >>>>> >>>>>> Cassandra has to flush the memtable occasionally, or the commit log >>>>>> grows without bounds. >>>>>> >>>>>> On 05/25/2017 03:42 AM, preetika tyagi wrote: >>>>>> >>>>>> Hi, >>>>>> >>>>>> I'm running Cassandra with a very small dataset so that the data can >>>>>> exist on memtable only. Below are my configurations: >>>>>> >>>>>> In jvm.options: >>>>>> >>>>>> -Xms4G >>>>>> -Xmx4G >>>>>> >>>>>> In cassandra.yaml, >>>>>> >>>>>> memtable_cleanup_threshold: 0.50 >>>>>> memtable_allocation_type: heap_buffers >>>>>> >>>>>> As per the documentation in cassandra.yaml, the >>>>>> *memtable_heap_space_in_mb* and *memtable_heap_space_in_mb* will be >>>>>> set of 1/4 of heap size i.e. 1000MB >>>>>> >>>>>> According to the documentation here (http://docs.datastax.com/en/c >>>>>> assandra/3.0/cassandra/configuration/configCassandra_yaml.ht >>>>>> ml#configCassandra_yaml__memtable_cleanup_threshold), the memtable >>>>>> flush will trigger if the total size of memtabl(s) goes beyond >>>>>> (1000+1000)*0.50=1000MB. >>>>>> >>>>>> Now if I perform several write requests which results in almost >>>>>> ~300MB of the data, memtable still gets flushed since I see sstables >>>>>> being >>>>>> created on file system (Data.db etc.) and I don't understand why. >>>>>> >>>>>> Could anyone explain this behavior and point out if I'm missing >>>>>> something here? >>>>>> >>>>>> Thanks, >>>>>> >>>>>> Preetika >>>>>> >>>>>> >>>>>> >>>>> >>>> >>> >> > -- Stefan Litsche | Mobile: +49 176 12759436 E-Mail: stefan.lits...@zalando.de
