[Gluster-users] Gluster on EC2 - how to replace failed EBS volume?

Olivier Nicole Olivier.Nicole at cs.ait.ac.th
Fri Oct 7 02:56:11 UTC 2011

Hi Jeff,

> "So I would not use any RAID on the machine, just have 8 independent
> disks and mount the 8 disks at eight locations:"
> Then your max file size is limited to the space available in each 
> disk/brick unless you stripe the data with Gluster.  I don't think 

Don was mentionning RAID 0, so yes, I consider stiping in Gluster
instead. It will give more flexibility, at the cost of a little speed.

To be more exact, the OP is using RAID 0 then replicate with Gluster.
Instead I said he may be better be doing the striping and replication
in Gluster. RAID 0 will not bring him much, excet headache when he
needs to rebuild a 1TB brick instead of a 125GB one (or if he ever
find himself in a situation where he has 2 machines with 4 disks each
instead of one machine with 8 disks).

> Gluster is a replacement for RAID0,1,5,10 it should layer on top of it 
> to provide more redundancy or speed.

The thing is RAID 0 is not provinding any redundancy, only problems :)

Note I did not mention any other RAID level. My plan is RAID 10
bricks, in Gluster replicate/striping.

> In distribute/replicate your max file size is your brick size is the 
> best case scenario.  250GB bricks = 250GB max file size.  What if your 
> brick already has 230GB used?  Now you max file size it 20G and you get 
> a full brick if you use it.  Create bigger bricks and you have lowered 
> your chance of having too big of a file and lowered your risk of getting 
> a full brick.
> I'm fairly new to gluster so if I'm wrong anyone can feel free to 
> correct me.  The only bad thing about big bricks is it makes it harder 
> to add more bricks since you want to add bricks of the same size if 
> possible.

You can always decide to have pretty "small" briks, like 250GB, and if
you have 1TB available on one machine, you create 4 briks on that
machine and let gluster stripe them for you. When your hard disk get
1.5TB, you have 6 briks, and that it. You add overhead, you add
fragmentation, but you ease-up your managment; it's all a balance of
pro's and cons' :)



> Jeff White
> Linux/Unix Systems Engineer
> University of Pittsburgh - CSSD
> Jaw171 at pitt.edu
> On 10/05/2011 10:45 PM, Olivier Nicole wrote:
> > Hi Don,
> >
> >> Thanks for your reply.  Can you explain what you mean by:
> >>
> >>> Instead of configuring your 8 disks in RAID 0, I would use JOBD and
> >>> let Gluster do the concatenation. That way, when you replace a disk,
> >>> you just have 125 GB to self-heal.
> > If I am not mistaken, RAID 0 provides no redundancy, it just
> > concatenates the 8 125GB disks together so they appear as one big 1TB
> > disk.
> >
> > So I would not use any RAID on the machine, just have 8 independent
> > disks and mount the 8 disks at eight locations:
> >
> > mount /dev/sda1 /
> > mount /dev/sdb1 /datab
> > mount /dev/sdc1 /datac
> > etc.
> >
> > The in gluster I would have the bricks
> >
> > server:/data
> > server:/datab
> > server:/datac
> > etc.
> >
> > If any disk (except the system disk) fails, you can simply fit in a
> > new disk and let gluster self-heal.
> >
> > Even if RAID 0 increases the disk throughput because it does stripping
> > (write different blocks to different disks), gluster does the same
> > more or less, each new file will end up in a different disk. So the
> > trhoughput should be close.
> >
> > The only disadvantage is that gluster will have some space overhead,
> > as it will create a replicate of the directory tree on each disk.
> >
> > I think that you should only use RAID with gluster when RAID provides
> > local redundancy (RAID 1 or above): in that case, when a disk fails,
> > gluster will not notice the problem, you swap to a new disk and let
> > RAID rebuild the information.
> >
> > Bests,
> >
> > Olivier

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