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https://issues.apache.org/jira/browse/KAFKA-188?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13475735#comment-13475735
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Jun Rao commented on KAFKA-188:
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Thanks for the patch. A couple of comments:
1. It's probably better to have a separate high watermark file per dir for
partitions assigned to it. That way, if a disk is damaged, we only lose the
high watermarks and the data for partitions on that disk.
2. About data balancing, in the normal case, assigning partitions in a
round-robin way to log dirs is fine. However, if a disk is damaged and is
replaced, initially there is no data on it. The round-robin approach would mean
that the newly replaced disk will always have fewer partitions that other
disks. The same issue can occur if a new dir is added in the configuration. An
alternative approach is to assign a new partition to the dir with the fewest
partitions, which would alleviate this issue.
> Support multiple data directories
> ---------------------------------
>
> Key: KAFKA-188
> URL: https://issues.apache.org/jira/browse/KAFKA-188
> Project: Kafka
> Issue Type: New Feature
> Reporter: Jay Kreps
> Attachments: KAFKA-188.patch
>
>
> Currently we allow only a single data directory. This means that a multi-disk
> configuration needs to be a RAID array or LVM volume or something like that
> to be mounted as a single directory.
> For a high-throughput low-reliability configuration this would mean RAID0
> striping. Common wisdom in Hadoop land has it that a JBOD setup that just
> mounts each disk as a separate directory and does application-level balancing
> over these results in about 30% write-improvement. For example see this claim
> here:
> http://old.nabble.com/Re%3A-RAID-vs.-JBOD-p21466110.html
> It is not clear to me why this would be the case--it seems the RAID
> controller should be able to balance writes as well as the application so it
> may depend on the details of the setup.
> Nonetheless this would be really easy to implement, all you need to do is add
> multiple data directories and balance partition creation over these disks.
> One problem this might cause is if a particular topic is much larger than the
> others it might unbalance the load across the disks. The partition->disk
> assignment policy should probably attempt to evenly spread each topic to
> avoid this, rather than just trying keep the number of partitions balanced
> between disks.
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