They can be and it depends on your compaction strategy :) On Sun, 6 Nov 2016 at 21:24 Ali Akhtar <ali.rac...@gmail.com> wrote:
> 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 > > > -- Ben Bromhead CTO | Instaclustr <https://www.instaclustr.com/> +1 650 284 9692 Managed Cassandra / Spark on AWS, Azure and Softlayer
