Re: Designing a cluster guide

[Stefan Priebe <s.priebe@profihost.ag>]

Hi Greg,

Am 17.05.2012 23:27, schrieb Gregory Farnum:
>> It mentions for example "Fast CPU" for the mds system. What does fast
>> mean? Just the speed of one core? Or is ceph designed to use multi core?
>> Is multi core or more speed important?
> Right now, it's primarily the speed of a single core. The MDS is
> highly threaded but doing most things requires grabbing a big lock.
> How fast is a qualitative rather than quantitative assessment at this
> point, though.
So would you recommand a fast (more ghz) Core i3 instead of a single 
xeon for this system? (price per ghz is better).

> It depends on what your nodes look like, and what sort of cluster
> you're running. The monitors are pretty lightweight, but they will add
> *some* load. More important is their disk access patterns — they have
> to do a lot of syncs. So if they're sharing a machine with some other
> daemon you want them to have an independent disk and to be running a
> new kernel&glibc so that they can use syncfs rather than sync. (The
> only distribution I know for sure does this is Ubuntu 12.04.)
Which kernel and which glibc version supports this? I have searched 
google but haven't found an exact version. We're using debian lenny 
squeeze with a custom kernel.

>> Regarding the OSDs is it fine to use an SSD Raid 1 for the journal and
>> perhaps 22x SATA Disks in a Raid 10 for the FS or is this quite absurd
>> and you should go for 22x SSD Disks in a Raid 6?
> You'll need to do your own failure calculations on this one, I'm
> afraid. Just take note that you'll presumably be limited to the speed
> of your journaling device here.
Yeah that's why i wanted to use a Raid 1 of SSDs for the journaling. Or 
is this still too slow? Another idea was to use only a ramdisk for the 
journal and backup the files while shutting down to disk and restore 
them after boot.

> Given that Ceph is going to be doing its own replication, though, I
> wouldn't want to add in another whole layer of replication with raid10
> — do you really want to multiply your storage requirements by another
> factor of two?
OK correct bad idea.

>> Is it more useful the use a Raid 6 HW Controller or the btrfs raid?
> I would use the hardware controller over btrfs raid for now; it allows
> more flexibility in eg switching to xfs. :)
OK but overall you would recommand running one osd per disk right? So 
instead of using a Raid 6 with for example 10 disks you would run 6 osds 
on this machine?

>> Use single socket Xeon for the OSDs or Dual Socket?
> Dual socket servers will be overkill given the setup you're
> describing. Our WAG rule of thumb is 1GHz of modern CPU per OSD
> daemon. You might consider it if you decided you wanted to do an OSD
> per disk instead (that's a more common configuration, but it requires
> more CPU and RAM per disk and we don't know yet which is the better
> choice).
Is there also a rule of thumb for the memory?

My biggest problem with ceph right now is the awful slow speed while 
doing random reads and writes.

Sequential read and writes are at 200Mb/s (that's pretty good for bonded 
dual Gbit/s). But random reads and write are only at 0,8 - 1,5 Mb/s 
which is def. too slow.

Stefan
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