[Gluster-users] how well will this work

[Joe Julian]

On 01/02/2013 03:37 AM, Jeff Darcy wrote:
> On 1/2/13 6:01 AM, Brian Candler wrote:
>> On Fri, Dec 28, 2012 at 10:14:19AM -0800, Joe Julian wrote:
>>> My volume would then look like
>>> gluster volume create replica 3
>>> server{1,2,3}:/data/glusterfs/vmimages/a/brick
>>> server{1,2,3}:/data/glusterfs/vmimages/b/brick
>>> server{1,2,3}:/data/glusterfs/vmimages/c/brick
>>> server{1,2,3}:/data/glusterfs/vmimages/d/brick
>> Aside: what is the reason for creating four multiple logical volumes/bricks
>> on the same node, and then combining them together using gluster
>> distribution?
> I'm not Joe, but I can think of two reasons why this might be a good idea.  One
> is superior fault isolation.  With a single concatenated or striped LV (i.e. no
> redundancy as with true RAID), a failure of any individual disk will appear as
> a failure of the entire brick, forcing *all* traffic to the peers.  With
> multiple LVs, that same failure will cause only 1/4 of the traffic to fail
> over.  The other reason is performance.  I've found that it's very hard to
> predict whether letting LVM schedule across disks or letting GlusterFS do so
> will perform better for any given workload, but IMX the latter tends to win
> slightly more often than not.
>
Fault isolation is, indeed, why I do that. I don't need any faster reads 
than my network will handle, so raid isn't going to help me there. When 
a drive fails, gluster's (mostly) been good about handling that failure 
transparently to my services.

>> Also, why are you combining all your disks into a single
>> volume group (clustervg), but then allocating each logical volume from only
>> a single disk within that VG?
> That part's a bit unclear to me as well.  There doesn't seem to be any
> immediate benefit, but perhaps it's more an issue of preparing for possible
> future change by adding an extra level of naming/indirection.  That way, if the
> LVs need to be reconfigured some day, the change will be pretty transparent to
> anything that was addressing them by ID anyway.
>
Aha! Because when a drive's in pre-failure I can pvmove the lv's onto 
the new drive, or onto the other drives temporarily.