[ovirt-users] Good practices

Mon Aug 7 18:43:29 UTC 2017

On Mon, Aug 7, 2017 at 6:41 PM, FERNANDO FREDIANI <fernando.frediani at upx.com
> wrote:

> Thanks for the clarification Erekle.
>
> However I get surprised with this way of operating from GlusterFS as it
> adds another layer of complexity to the system (either a hardware or
> software RAID) before the gluster config and increase the system's overall
> costs.
>

It does, but with HW based RAID it's not a big deal. The complexity is all
the stripe size math... which I personally don't like to calculate.

> An important point to consider is: In RAID configuration you already have
> space 'wasted' in order to build redundancy (either RAID 1, 5, or 6). Then
> when you have GlusterFS on the top of several RAIDs you have again more
> data replicated so you end up with the same data consuming more space in a
> group of disks and again on the top of several RAIDs depending on the
> Gluster configuration you have (in a RAID 1 config the same data is
> replicated 4 times).
>
> Yet another downside of having a RAID (specially RAID 5 or 6) is that it
> reduces considerably the write speeds as each group of disks will end up
> having the write speed of a single disk as all other disks of that group
> have to wait for each other to write as well.
>
> Therefore if Gluster already replicates data why does it create this big
> pain you mentioned if the data is replicated somewhere else, can still be
> retrieved to both serve clients and reconstruct the equivalent disk when it
> is replaced ?
>

I think it's a matter of how fast you can replace a disk (over a long
weekend?), how reliably you can do it (please, don't pull the wrong disk!
I've seen it happening too many times!) and how much of a performance hit
are you willing to accept while in degraded mode (and how long it took to
detect it. HDDs, unlike SSDs, die slowly. At least when SSD dies, it dies a
quick and determined death. HDDs may accumulate errors and errors and still
function).
Y.

Fernando
>
> On 07/08/2017 10:26, Erekle Magradze wrote:
>
> Hi Frenando,
>
> Here is my experience, if you consider a particular hard drive as a brick
> for gluster volume and it dies, i.e. it becomes not accessible it's a huge
> hassle to discard that brick and exchange with another one, since gluster
> some tries to access that broken brick and it's causing (at least it cause
> for me) a big pain, therefore it's better to have a RAID as brick, i.e.
> have RAID 1 (mirroring) for each brick, in this case if the disk is down
> you can easily exchange it and rebuild the RAID without going offline, i.e
> switching off the volume doing brick manipulations and switching it back on.
>
> Cheers
>
> Erekle
>
> On 08/07/2017 03:04 PM, FERNANDO FREDIANI wrote:
>
> For any RAID 5 or 6 configuration I normally follow a simple gold rule
> which gave good results so far:
> - up to 4 disks RAID 5
> - 5 or more disks RAID 6
>
> However I didn't really understand well the recommendation to use any RAID
> with GlusterFS. I always thought that GlusteFS likes to work in JBOD mode
> and control the disks (bricks) directlly so you can create whatever
> distribution rule you wish, and if a single disk fails you just replace it
> and which obviously have the data replicated from another. The only
> downside of using in this way is that the replication data will be flow
> accross all servers but that is not much a big issue.
>
> Anyone can elaborate about Using RAID + GlusterFS and JBOD + GlusterFS.
>
> Thanks
> Regards
> Fernando
>
> On 07/08/2017 03:46, Devin Acosta wrote:
>
>
> Moacir,
>
> I have recently installed multiple Red Hat Virtualization hosts for
> several different companies, and have dealt with the Red Hat Support Team
> in depth about optimal configuration in regards to setting up GlusterFS
> most efficiently and I wanted to share with you what I learned.
>
> In general Red Hat Virtualization team frowns upon using each DISK of the
> system as just a JBOD, sure there is some protection by having the data
> replicated, however, the recommendation is to use RAID 6 (preferred) or
> RAID-5, or at least RAID-1 at the very least.
>
> Here is the direct quote from Red Hat when I asked about RAID and Bricks:
>
> *"A typical Gluster configuration would use RAID underneath the bricks.
> RAID 6 is most typical as it gives you 2 disk failure protection, but RAID
> 5 could be used too. Once you have the RAIDed bricks, you'd then apply the
> desired replication on top of that. The most popular way of doing this
> would be distributed replicated with 2x replication. In general you'll get
> better performance with larger bricks. 12 drives is often a sweet spot.
> Another option would be to create a separate tier using all SSD’s.” *
>
> *In order to SSD tiering from my understanding you would need 1 x NVMe
> drive in each server, or 4 x SSD hot tier (it needs to be distributed,
> replicated for the hot tier if not using NVME). So with you only having 1
> SSD drive in each server, I’d suggest maybe looking into the NVME option. *
>
> *Since your using only 3-servers, what I’d probably suggest is to do (2
> Replicas + Arbiter Node), this setup actually doesn’t require the 3rd
> server to have big drives at all as it only stores meta-data about the
> files and not actually a full copy. *
>
> *Please see the attached document that was given to me by Red Hat to get
> more information on this. Hope this information helps you.*
>
>
> --
>
> Devin Acosta, RHCA, RHVCA
> Red Hat Certified Architect
>
> On August 6, 2017 at 7:29:29 PM, Moacir Ferreira (
> moacirferreira at hotmail.com) wrote:
>
> I am willing to assemble a oVirt "pod", made of 3 servers, each with 2 CPU
> sockets of 12 cores, 256GB RAM, 7 HDD 10K, 1 SSD. The idea is to use
> GlusterFS to provide HA for the VMs. The 3 servers have a dual 40Gb NIC and
> a dual 10Gb NIC. So my intention is to create a loop like a server triangle
> using the 40Gb NICs for virtualization files (VMs .qcow2) access and to
> move VMs around the pod (east /west traffic) while using the 10Gb
> interfaces for giving services to the outside world (north/south traffic).
>
>
> This said, my first question is: How should I deploy GlusterFS in such
> oVirt scenario? My questions are:
>
>
> 1 - Should I create 3 RAID (i.e.: RAID 5), one on each oVirt node, and
> then create a GlusterFS using them?
>
> 2 - Instead, should I create a JBOD array made of all server's disks?
>
> 3 - What is the best Gluster configuration to provide for HA while not
> consuming too much disk space?
>
> 4 - Does a oVirt hypervisor pod like I am planning to build, and the
> virtualization environment, benefits from tiering when using a SSD disk?
> And yes, will Gluster do it by default or I have to configure it to do so?
>
>
> At the bottom line, what is the good practice for using GlusterFS in small
> pods for enterprises?
>
>
> You opinion/feedback will be really appreciated!
>
> Moacir
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