raid5 + HDFS

[Jim Dowling <jdowling@sics.se>]

Hi
I am interested in combining BtrFS RAID-5 with erasure-coded replication 
for HDFS. We have an implementation of Reed-Solomon replication for our 
HDFS distribution called HopsFS (www.hops.io).

Some of the nice features of HDFS that make it suitable are:
* not many small files
* not excessive snapshotting
* can probably manage disks being close to capacity, as its globally 
visible and blocks can be re-balanced in HDFS

What BtrFS could give us:
* stripping within a DataNode using RAID-5
* higher throughput read/write for HDFS clients over 10 GbE without 
losing data locality (others are looking at stripping blocks over many 
different nodes).
* true snapshotting for HDFS by providing rollback of HDFS blocks

I am concerned (actually, excited!) that RAID-5 is not production ready. 
Any guidelines on how mature it is, since the big PR in Feb/March 2015?
What about scrubbing for RAID-5?

Is there anything else I should know?

Btw, here are some links i found about RAID-5:


UREs are not as common on commodity disks - RAID-5 is safer than assumed:
*<https://www.high-rely.com/blog/using-raid5-means-the-sky-is-falling/>https://www.high-rely.com/blog/using-raid5-means-the-sky-is-falling/* 


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Btrfs Linux 4.1 with 5 10K RPM spinning disks:

<http://www.phoronix.com/scan.php?page=article&item=btrfs-raid015610-linux41&num=1>http://www.phoronix.com/scan.php?page=article&item=btrfs-raid015610-linux41&num=1

*Raid*-*5 *Results: ~250 MB/s for sequential reads. 559 MB/s for 
sequential writes.

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