On 2016-02-17 11:07, Christian Balzer wrote:
On Wed, 17 Feb 2016 10:04:11 +0100 Piotr Wachowicz wrote:
> > Let's consider both cases:
> > Journals on SSDs - for writes, the write operation returns right
> > after data lands on the Journal's SSDs, but before it's written to
> > the backing HDD. So, for writes, SSD journal approach should be
> > comparable to having a SSD cache tier.
> Not quite, see below.
>
>
Could you elaborate a bit more?
Are you saying that with a Journal on a SSD writes from clients,
before
they can return from the operation to the client, must end up on both
the
SSD (Journal) *and* HDD (actual data store behind that journal)?
No, your initial statement is correct.
However that burst of speed doesn't last indefinitely.
Aside from the size of the journal (which is incidentally NOT the most
limiting factor) there are various "filestore" parameters in Ceph, in
particular the sync interval ones.
There was a more in-depth explanation by a developer about this in this
ML,
try your google-foo.
For short bursts of activity, the journal helps a LOT.
If you send a huge number of for example 4KB writes to your cluster,
the
speed will eventually (after a few seconds) go down to what your
backing
storage (HDDs) are capable of sustaining.
> (Which SSDs do you plan to use anyway?)
>
Intel DC S3700
Good choice, with the 200GB model prefer the 3700 over the 3710 (higher
sequential write speed).
Hi All,
I am looking at using PCI-E SSDs as journals in our (4) Ceph OSD nodes,
each of which has 6 4TB SATA drives within. I had my eye on these:
400GB Intel P3500 DC AIC SSD, HHHL PCIe 3.0
but reading through this thread, it might be better to go with the P3700
given the improved iops. So a couple of questions.
* Are the PCI-E versions of these drives different in any other way than
the interface?
* Would one of these as a journal for 6 4TB OSDs be overkill
(connectivity is 10GE, or will be shortly anyway), would the SATA S3700
be sufficient?
Given they're not hot-swappable, it'd be good if they didn't wear out in
6 months too.
I realise I've not given you much to go on and I'm Googling around as
well, I'm really just asking in case someone has tried this already and
has some feedback or advice..
Thanks! :)
Stephen
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Stephen Harker
Chief Technology Officer
The Positive Internet Company.
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