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From "Tupshin Harper (JIRA)" <j...@apache.org>
Subject [jira] Commented: (CASSANDRA-1214) Make standard IO the default
Date Tue, 29 Jun 2010 22:13:53 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-1214?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12883726#action_12883726
] 

Tupshin Harper commented on CASSANDRA-1214:
-------------------------------------------

I am strongly in favor of defaults that are as flexible and stable as possible. If it is hard
for even a relatively small percentage of users to get stable performance with mmap, then
I would agree that the default should be standard I/O. There should then be a Cassandra Tuning
wiki page that include a mmap discussion.

That said, I also agree that it is worth doing the native code work to get mmap more stable
with larger datasets and/or smaller machines.

> Make standard IO the default
> ----------------------------
>
>                 Key: CASSANDRA-1214
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1214
>             Project: Cassandra
>          Issue Type: Bug
>    Affects Versions: 0.7
>            Reporter: James Golick
>
> The way mmap()'d IO is handled in cassandra is dangerous. It allocates potentially massive
buffers without any care for bounding the total size of the program's buffers. As the node's
dataset grows, this *will* lead to swapping and instability.
> This is a dangerous and wrong default for a couple of reasons.
> 1) People are likely to test cassandra with the default settings. This issue is insidious
because it only appears when you have sufficient data in a certain node, there is absolutely
no way to control it, and it doesn't at all respect the memory limits that you give to the
JVM.
> That can all be ascertained by reading the code, and people should certainly do their
homework, but nevertheless, cassandra should ship with sane defaults that don't break down
when you cross some magic unknown threshold.
> 2) It's deceptive. Unless you are extremely careful with capacity planning, you will
get bit by this. Most people won't really be able to use this in production, so why get them
excited about performance that they can't actually have?

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