blob size considerations

[nikolay.dimitrov@gmail.com]

Hi everyone,

We are evaluating Scylla and were wondering what the limitations/considerations about using blobs are.

Basically, we want to store data which is usually 200-300KB, but can get up to 15-20MB in size.

From the Cassandra documentation http://docs.datastax.com/en/cql/3.3/cql/cql_reference/blob_r.html: 

The maximum theoretical size for a blob is 2 GB. The practical limit on blob size, however, is less than 1 MB.

What is the case with Scylla, is the same situation?

Thanks in advance to those who can help!

[Avi Kivity]

Hi,

Both the theoretical and practical limits are the same in Cassandra and Scylla. 20 MB blobs will work, but you may have to experiment to get the client concurrency that still gives good performance. A good point to start is 2*(total number of Scylla cores in a data center).

We plan to improve large blob support in the future so that you will get good performance without client-side tuning.

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[Nikolay Dimitrov]

Hi Avi,

Thanks for the quick response!

Ok, we'll give it a go and see what we can get performance-wise.

Cheers

On 19 June 2017 at 19:34, Avi Kivity <a...@scylladb.com> wrote:

Hi,

Both the theoretical and practical limits are the same in Cassandra and Scylla. 20 MB blobs will work, but you may have to experiment to get the client concurrency that still gives good performance. A good point to start is 2*(total number of Scylla cores in a data center).

We plan to improve large blob support in the future so that you will get good performance without client-side tuning.

On 06/19/2017 01:23 PM, nikolay....@gmail.com wrote:

Hi everyone,

We are evaluating Scylla and were wondering what the limitations/considerations about using blobs are.

Basically, we want to store data which is usually 200-300KB, but can get up to 15-20MB in size.

From the Cassandra documentation http://docs.datastax.com/en/cql/3.3/cql/cql_reference/blob_r.html: 

The maximum theoretical size for a blob is 2 GB. The practical limit on blob size, however, is less than 1 MB.

What is the case with Scylla, is the same situation?

Thanks in advance to those who can help!

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[Glauber Costa]

On Mon, Jun 19, 2017 at 6:34 AM, Avi Kivity <a...@scylladb.com> wrote:
> Hi,
>
>
> Both the theoretical and practical limits are the same in Cassandra and
> Scylla. 20 MB blobs will work, but you may have to experiment to get the
> client concurrency that still gives good performance. A good point to start
> is 2*(total number of Scylla cores in a data center).
>
>
> We plan to improve large blob support in the future so that you will get
> good performance without client-side tuning.

by default a single mutation is restricted to 16MB - which is half of
the commitlog segment size.

That said, you can increase the segment size - but that has to be done
manually in the yaml file.

> https://groups.google.com/d/msgid/scylladb-users/8cfd8c97-49a6-de24-97c8-b12d13edab8e%40scylladb.com.

[nikolay.dimitrov@gmail.com]

Thanks for the info Glauber, will have that in mind too!

[nikolay.dimitrov@gmail.com]

Hi again,

I'm experimenting now with the C++ DataStax driver for Cassandra and everything works well except when I try to insert a blob bigger than 16mb.

The result is a time out ("Request timed out") and it waits for about 13 seconds before returning that error.

I have set:

write_request_timeout_in_ms: 20000

request_timeout_in_ms: 20000

Not sure where those 13 seconds come from. Also, if I insert a blob that is exactly 16MB, it goes in really fast (a few milliseconds).

If I increase the size with just a few bytes, it gets stuck with the above timeout.

Any ideas? 

[Tomer Sandler]

I think this is why... 

by default a single mutation is restricted to 16MB - which is half of the commitlog segment size.

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ScyllaDB

(Sent from my android)

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[Nikolay Dimitrov]

Thanks for the reply, Tomer. 

Is there any config option to increase that or it's a compile-time constant? 

Is it a good idea to increase it, say to 32MB?

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[Glauber Costa]

On Fri, Jun 23, 2017 at 5:10 AM, Nikolay Dimitrov
<nikolay....@gmail.com> wrote:
> Thanks for the reply, Tomer.
>
> Is there any config option to increase that or it's a compile-time constant?
> Is it a good idea to increase it, say to 32MB?
>

This is an option in the yaml file:

commitlog_segment_size_in_mb: 32

A single mutation can only go to half of that.

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[Nikolay Dimitrov]

Great, thanks for the response Glauber!

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[Tzach Livyatan]

On Fri, Jun 23, 2017 at 11:55 AM, Tomer Sandler <to...@scylladb.com> wrote:

I think this is why... 

by default a single mutation is restricted to 16MB - which is half of the commitlog segment size.

This need to be better documented.

I assume the 1/2 of commitlog_segment_size_in_mb comes from this line:

https://github.com/scylladb/scylla/blob/master/db/commitlog/commitlog.cc#L944

right?

 

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[Glauber Costa]

Hi

did you manage to get this working ?

On Fri, Jun 23, 2017 at 9:33 AM, Nikolay Dimitrov

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[Nikolay Dimitrov]

Hi Glauber,

Yes, I've set:

commitlog_segment_size_in_mb: 64 

and managed to push blobs up to 32MB in size.

Regards

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