impact of update operation to read operation

[Jiaan Zeng <l....@gmail.com>]

Hi All,

I am using Cassandra 1.2.4. I wonder if update operation has
*permanent* impacts on read operation. Below is the scenario.

Previously, a read only workload runs against one column family and
has 4000 qps. Later, a read-update mixed workload  runs against the
same column family. After that, the read only workload runs again but
it could not get 4000 qps, only get 3500 qps. After a manual major
compaction is issued through command line, the read only workload gets
3600 qps which seems to tell major compaction does not help much.

Did anyone have similar experiences? Any idea why this is happening? Thanks.

-- 
Regards,
Jiaan

Re: impact of update operation to read operation

[Benedict Elliott Smith <be...@datastax.com>]

Also, a major compaction doesn't flush the memtable. If the memtable is
still full, reads may take slightly longer as they may have to be merged
with any on-disk data before being served.


On 10 February 2014 21:18, Tupshin Harper <tu...@tupshin.com> wrote:

> You don't mention disks and RAM,  but I would assume that the additional
> data meant that you could now cache a lower percentage and that you have to
> seek on disk more often.
>
> -Tupshin
> On Feb 10, 2014 4:14 PM, "Jiaan Zeng" <l....@gmail.com> wrote:
>
>> Hi All,
>>
>> I am using Cassandra 1.2.4. I wonder if update operation has
>> *permanent* impacts on read operation. Below is the scenario.
>>
>> Previously, a read only workload runs against one column family and
>> has 4000 qps. Later, a read-update mixed workload  runs against the
>> same column family. After that, the read only workload runs again but
>> it could not get 4000 qps, only get 3500 qps. After a manual major
>> compaction is issued through command line, the read only workload gets
>> 3600 qps which seems to tell major compaction does not help much.
>>
>> Did anyone have similar experiences? Any idea why this is happening?
>> Thanks.
>>
>> --
>> Regards,
>> Jiaan
>>
>

Re: impact of update operation to read operation

[Tupshin Harper <tu...@tupshin.com>]

You don't mention disks and RAM,  but I would assume that the additional
data meant that you could now cache a lower percentage and that you have to
seek on disk more often.

-Tupshin
On Feb 10, 2014 4:14 PM, "Jiaan Zeng" <l....@gmail.com> wrote:

> Hi All,
>
> I am using Cassandra 1.2.4. I wonder if update operation has
> *permanent* impacts on read operation. Below is the scenario.
>
> Previously, a read only workload runs against one column family and
> has 4000 qps. Later, a read-update mixed workload  runs against the
> same column family. After that, the read only workload runs again but
> it could not get 4000 qps, only get 3500 qps. After a manual major
> compaction is issued through command line, the read only workload gets
> 3600 qps which seems to tell major compaction does not help much.
>
> Did anyone have similar experiences? Any idea why this is happening?
> Thanks.
>
> --
> Regards,
> Jiaan
>