Re: OSD nodes with >=8 spinners, SSD-backed journals, and their performance impact

[Florian Haas <florian@hastexo.com>]

On 01/14/2013 06:34 PM, Gregory Farnum wrote:
> On Mon, Jan 14, 2013 at 6:09 AM, Florian Haas <florian@hastexo.com> wrote:
>> Hi Mark,
>>
>> thanks for the comments.
>>
>> On Mon, Jan 14, 2013 at 2:46 PM, Mark Nelson <mark.nelson@inktank.com> wrote:
>>> Hi Florian,
>>>
>>> Couple of comments:
>>>
>>> "OSDs use a write-ahead mode for local operations: a write hits the journal
>>> first, and from there is then being copied into the backing filestore."
>>>
>>> It's probably important to mention that this is true by default only for
>>> non-btrfs file systems.  See:
>>>
>>> http://ceph.com/wiki/OSD_journal
>>
>> I am well aware of that, but I've yet to find a customer (or user)
>> that's actually willing to entrust a production cluster with several
>> hundred terabytes of data to btrfs. :) Besides, the whole post is
>> about whether or not to use dedicated SSD block devices for OSD
>> journals, and if you're tossing everything into btrfs you've already
>> made the decision to use in-filestore journals.
> 
> That is absolutely not the case. btrfs works just fine with an
> external journal on SSD or whatever else; what made you think
> otherwise?

A misunderstanding on my part. Also, I was overly broad in my comment.
What I really meant to say was that if I'm using a btrfs filestore, and
a separate dedicated block device for the journal, then the journaling
mode is write-ahead and not parallel.

Which was a wrong assumption on my part, as an external journal combined
with a btrfs filestore seems to support parallel journaling mode just
fine. For some reason I had supposed the journal had to be in the same
btrfs as the filestore for this to work.

Sorry for the confusion.

Cheers,
Florian