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It's generally recommended that cassandra use two separate disks: one for the commit log and the other for everything else.

However, in what appears to be a recent update to the configuration guidelines, the following phrase appears:

For SSDs it is recommended that both commit logs and SSTables are on the same mount point.

Can anyone explain why it's recommended to only use one disk if it's SSD? Thanks.

David Semeria
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2 Answers2

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The reason you use separate disk for the commit log when using regular hard drives is so you only do sequential writes on the commit log hard drive.

Other cassandra activity, such as reading/compaction etc, will affect cause random access on the other disk, not your commit log meaning that writes to your commit log will be very fast.

For SSD random access is as performant as sequential access so there is no need to prevent it to keep your commit log happy.

simo
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Christopher Batey
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  • Yes, but since Cassandra is multi-threaded I would have assumed that having a dedicated disk for the commit log means commit log appends would never have to wait for the disk to be free. I can only assume that SSDs are so fast it makes such wait times negligible... – David Semeria Sep 17 '13 at 08:06
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I think that you still would want separate disks in many cases due to SSD Write Amplification. The idea is that a separate commit log SSD would suffer no WA, which is good since it should be the fastest write location you have.

8forty
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