tl;dr? I just want to know if updates are bad for performance, and if so, for how long.
On Mon, Nov 7, 2016 at 10:23 AM, Ben Bromhead <b...@instaclustr.com> wrote: > Check out https://wiki.apache.org/cassandra/WritePathForUsers for the > full gory details. > > On Sun, 6 Nov 2016 at 21:09 Ali Akhtar <ali.rac...@gmail.com> wrote: > >> How long does it take for updates to get merged / compacted into the main >> data file? >> >> On Mon, Nov 7, 2016 at 5:31 AM, Ben Bromhead <b...@instaclustr.com> wrote: >> >> To add some flavor as to how the commitlog implementation is so quick. >> >> It only flushes to disk every 10s by default. So writes are effectively >> done to memory and then to disk asynchronously later on. This is generally >> accepted to be OK, as the write is also going to other nodes. >> >> You can of course change this behavior to flush on each write or to skip >> the commitlog altogether (danger!). This however will change how "safe" >> things are from a durability perspective. >> >> On Sun, Nov 6, 2016, 12:51 Jeff Jirsa <jeff.ji...@crowdstrike.com> wrote: >> >> Cassandra writes are particularly fast, for a few reasons: >> >> >> >> 1) Most writes go to a commitlog (append-only file, written >> linearly, so particularly fast in terms of disk operations) and then pushed >> to the memTable. Memtable is flushed in batches to the permanent data >> files, so it buffers many mutations and then does a sequential write to >> persist that data to disk. >> >> 2) Reads may have to merge data from many data tables on disk. >> Because the writes (described very briefly in step 1) write to immutable >> files, updates/deletes have to be merged on read – this is extra effort for >> the read path. >> >> >> >> If you don’t do much in terms of overwrites/deletes, and your partitions >> are particularly small, and your data fits in RAM (probably mmap/page cache >> of data files, unless you’re using the row cache), reads may be very fast >> for you. Certainly individual reads on low-merge workloads can be < 0.1ms. >> >> >> >> - Jeff >> >> >> >> *From: *Vikas Jaiman <er.vikasjai...@gmail.com> >> *Reply-To: *"user@cassandra.apache.org" <user@cassandra.apache.org> >> *Date: *Sunday, November 6, 2016 at 12:42 PM >> *To: *"user@cassandra.apache.org" <user@cassandra.apache.org> >> *Subject: *Are Cassandra writes are faster than reads? >> >> >> >> Hi all, >> >> >> >> Are Cassandra writes are faster than reads ?? If yes, why is this so? I >> am using consistency 1 and data is in memory. >> >> >> >> Vikas >> >> -- >> Ben Bromhead >> CTO | Instaclustr <https://www.instaclustr.com/> >> +1 650 284 9692 >> Managed Cassandra / Spark on AWS, Azure and Softlayer >> >> >> -- > Ben Bromhead > CTO | Instaclustr <https://www.instaclustr.com/> > +1 650 284 9692 > Managed Cassandra / Spark on AWS, Azure and Softlayer >