Raid-0 or not?

[tepheikk]

Hi.

I'm currently reviewing one MogileFS installation. It has been done
with RAID-0. Files that are stored are images 200kb-800kb. They are
also served via HTTP. Basically there's lots of concurrent requests.

So now I'm thinking is Raid-0 really the best and most optimal
solution? If instead there would be two separate disks shouldn't there
be much better concurrency and "random" seek performance? There's like
10-20 million requests per day to the farm. Any opinions? Should a get
rid of Raid-0?

Also there's one another question. How to do it? There's lots of spare
space in the farm (3 computers). So basically I'll just drain one
node, repartition the disks and add it back to farm. Should it
rebalance automatically?

How does the replication really work. Is it done per machine or per
disk? So it there's a file that has min_dev_count=2 it's copied to two
separate machines not two separate disks?

Best regards
Tero Heikkinen

[sanados]

that is more a philosophie :)

i like to have plain disks.
Easy to replace, easy to setup.
And scalable because of mogilefs.
When i run into performance issues i just increase mindevcount and let
mogilefs rebalance and fsck.

rebalance:
root|srv079 +>mogadm settings list
enable_rebalance = 0

enable:
mogadm settings set enable_rebalance 1

replication TRIES to replicate every File over to 2hosts. (it will tell
you when there are not enough hosts to store the files, keep an eye at
the size of your logfiles )
When you have a mindev of 3 the files are stored at least on 2 different
hosts.

lg
Jürgen

[pion]

For your case it's seek, not read performance that's the important
factor. That means that you should definitely get rid of the raid0,
and run the harddrives are two devices in mogile.

With raid0 both harddrives would be busy with the same request, but
two harddrives you can ship one request to each harddrive, as mogile
should keep track of what harddrive got the most IO load.

Raid0 is only beneficial if you need large ammount of data per request