Using RAID 0 vs multiple data paths after commit #10461


(Scott-3) #1

Hello all,
I currently have 2 servers, each with 128GB RAM and 2 HDDs in a RAID 1, and
10 SDDs. I was instructed to create a ELK stack on these servers, using
the SDDs as separate disks and pointing ES to each as a path.data config.
The reasoning behind this was to avoid the speed hit from the RAID
controller, as well as flexibility. My plan was to have 1 server have a
single instance of ES with 31GB RAM and the rest of the memory for Kibana,
Logstash, Redis, and the 2nd server to have 2 instances of ES with 31 GB
each. When I researched about RAID vs the multiple data paths, everybody
seems to recommend not using RAID (or using RAID 0), but I fail to see how
multiple data paths will give flexibility. Up until today, I was under the
impression that ES stripes the data anyway, so it'll be like a RAID 0
(except with a lot more work on the fstab side of things). If one disk
went down, you'd lose the whole node since it didn't care about where it
placed the data. Now, however, I read the commit #10461 on github and it
seems to indicate that the code was changed to allow a single path for each
shard? If that's the case, and I have 3 shards + 1 replica each (because
we have 3 nodes), how does this utilize all 10 SDDs? If this is NOT the
case, the only real data redundancy and resiliency is still via the
replicas, correct? So it doesn't really matter RAID 0 vs multiple data
paths? Can anybody shed some light on this issue? I appreciate any and
all help!

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(Nik Everett) #2

I'm RAID 0 all the way. The striping is much more complete then ES's
path.data and operations is more used to the tool around it. Software raid
in linux is fine for this. We only do two disks in RAID 0 though because we
don't like the increased failure chance. So 10 in RAID 0 is a bit much. 10
disks isn't where our sweet spot is for ES so I haven't thought about it.

On Mon, May 18, 2015 at 11:20 AM, Scott sunglee2@gmail.com wrote:

Hello all,
I currently have 2 servers, each with 128GB RAM and 2 HDDs in a RAID 1,
and 10 SDDs. I was instructed to create a ELK stack on these servers,
using the SDDs as separate disks and pointing ES to each as a path.data
config. The reasoning behind this was to avoid the speed hit from the RAID
controller, as well as flexibility. My plan was to have 1 server have a
single instance of ES with 31GB RAM and the rest of the memory for Kibana,
Logstash, Redis, and the 2nd server to have 2 instances of ES with 31 GB
each. When I researched about RAID vs the multiple data paths, everybody
seems to recommend not using RAID (or using RAID 0), but I fail to see how
multiple data paths will give flexibility. Up until today, I was under the
impression that ES stripes the data anyway, so it'll be like a RAID 0
(except with a lot more work on the fstab side of things). If one disk
went down, you'd lose the whole node since it didn't care about where it
placed the data. Now, however, I read the commit #10461 on github and it
seems to indicate that the code was changed to allow a single path for each
shard? If that's the case, and I have 3 shards + 1 replica each (because
we have 3 nodes), how does this utilize all 10 SDDs? If this is NOT the
case, the only real data redundancy and resiliency is still via the
replicas, correct? So it doesn't really matter RAID 0 vs multiple data
paths? Can anybody shed some light on this issue? I appreciate any and
all help!

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(system) #3