Thanks again for all of your input. I agree with your assessment -- in our
cluster we avg 40ms for a 4k write. That's why we're adding the SSDs -- you
just can't run a proportioned RBD service without them.
I'll definitely give bcache a try in my test setup, but more reading has kinda
tempered my expectations -- the rate of oopses and hangs on the bcache ML seems
a bit high. And a 3.14 kernel would indeed still be a challenge on our RHEL6
boxes.
Cheers, Dan
September 4 2014 8:47 PM, "Robert LeBlanc" wrote:
You should be able to use any block device in a bcache device. Right now, we
are OK losing one SSD and it takes out 5 OSDs. We would rather have twice the
cache. Our opinion may change in the future. We wanted to keep as much overhead
as low as possible. I think we may spend the extra on heavier duty SSDs; less
overhead not having a mirror, and less SSD drives allow us to put in more high
capacity spindles in each host (we have a density need). We still have to test
how much cache is optimal. I'm not sure how much 1% SSD will help and if 2%
will make any difference, etc. That's why we need to test.
My reasoning is that if we can buffer the writes, hopefully we can write to the
spindles in a more linearly manner and give reads a better chance of getting
serviced faster. My theory is that with all the caching already happening at
KVM, etc that 1% writeback will be much more useful than 1% read cache because
the reads at the OSD level will be cache misses anyway because they will be
cold pages.
Write behind is really our target, reads can be serviced from cache a good
portion of the time, but writes have to always hit a disk (our current disk
system is about 33% read and 66% writes), in the Ceph case (and in our config)
three disks. When you add the latency from all the levels, network, kernel,
buffers, disk, etc it adds up to a lot. If you are always doing large
transfers, then it won't be too noticeable because bandwidth helps outweigh the
latency. But when we are dealing with thousands of VMs all doing very small
things like writing a couple of lines to a log, reading/writing some database
pages, etc, the latency just kills the performance in a big way. Reads aren't
too bad because only the primary OSD has to service the request, but on writes
all three OSDs have to acknowledge the write. So I'm trying to get the absolute
best write performance as I can. If I can reduce a 10 millisecond write to disk
to 1 ms, then I've saved about 18 ms in the transaction (primary OSD write plus
parallel write to secondary OSDs presumably).
With bcache, I'd even like to get rid of the journal writes in a test case.
Since it is hitting SSD and then being serialized to the backing disk anyway by
bcache, it seems that the journal is just a double write penalty. Then it will
be time to tackle the network latency, can't wait to get our Infiniband gear to
test AccelIO with Ceph.
I'm interested to see what you decide to do and what your results are.
On Thu, Sep 4, 2014 at 12:12 PM, Dan Van Der Ster wrote:
I've just been reading the bcache docs. It's a pity the mirrored writes
aren't implemented yet. Do you know if you can use an md RAID1 as a cache dev?
And is the graceful failover from wb to writethrough actually working without
data loss?
Also, write behind sure would help the filestore, since I'm pretty sure
the same 4k blocks are being overwritten many times (from our RBD clients).
Cheers, Dan
On Sep 4, 2014 7:44 PM, Robert LeBlanc wrote:
So far it was worked really well, we can raise/lower/disable/enable the cache
in realtime and watch how the load and traffic changes. There has been some
positive subjective results, but definitive results are still forth coming.
bcache on CentOS 7 was not easy, makes me wish we were running Debian or
Ubuntu. If there are enough reasons to train our admins on Debian/Ubuntu in
addition to learning CentOS7 for customer facing boxes, we may move that way
for Ceph and OpenStack, but I'm not sure how Red Hat purchasing Inktank will
shift the development from Debian/Ubuntu, so we don't want to make any big
changes until we have a better idea of what the future looks like. I think the
Enterprise versions of Ceph (n-1 or n-2) will be a bit too old from where we
want to be, which I'm sure will work wonderfully on Red Hat, but how will n.1,
n.2 or n.3 run?
Robert LeBlanc
On Thu, Sep 4, 2014 at 11:22 AM, Dan Van Der Ster wrote:
Hi Robert,
That's actually a pretty good idea, since bcache would also accelerate
the filestore flushes and leveldb. I actually wonder if an SSD-only pool would
even be faster than such a setup... probably not.
We're using an ancient enterprise n distro, so it will be a bit of a
headache to get the right kernel, etc .. But my colleague is planning to use
bcache to accelerate our hypervisors' ephemeral storage, so I guess that's a
solved problem.
Hmm...
Thanks!
Dan
On Sep 4, 2014 6:42 PM, Robert LeBlanc wrote:
We are still pretty early on in our testing of how to best use SSDs as well.
What we are trying right now, for some of the reasons you mentioned already, is
to use bcache as a cache for both journal and data. We have 10 spindles in our
boxes with 2 SSDs. We created two bcaches (one for each SSD) and put five
spindles behind it with the journals as just files on the spindle (because it
is hot, it should stay in SSD). This should have the advantage that if the SSD
fails, it could automatically fail to write-through mode (although I don't
think it will help if the SSD suddenly fails). However, it seems that if any
part of the journal is lost, the OSD is toast and needs to be rebuilt. Bcache
was appealing to us because one SSD could front multiple backend disks and make
the most efficient use of the SSD, it also has write around for large
sequential writes so that cache is not evicted for large sequential writes
which spindles are good at. Since we have a high read cache hit from KVM and
other layers, this is primary intended to help accelerate writes more than
reads (we are also more write heavy in our environment).
So far it seems to help, but we are going to start more in-depth testing soon.
One drawback is that bcache devices don't seem to like partitions, so we have
created the OSDs manually instead if using ceph-deploy.
I too am interested with other's experience with SSD and trying to
cache/accelerate Ceph. I think the Caching pool in the long run will be the
best option, but it can still use some performance tweaking with small reads
before it will be really viable for us.
Robert LeBlanc
On Thu, Sep 4, 2014 at 10:21 AM, Dan Van Der Ster wrote: Dear Cephalopods,
In a few weeks we will receive a batch of 200GB Intel DC S3700’s to augment
our cluster, and I’d like to hear your practical experience and discuss options
how best to deploy these.
We’ll be able to equip each of our 24-disk OSD servers with 4 SSDs, so they
will become 20 OSDs + 4 SSDs per server. Until recently I’ve been planning to
use the traditional deployment: 5 journal partitions per SSD. But as SSD-day
approaches, I growing less comfortable with the idea of 5 OSDs going down every
time an SSD fails, so perhaps there are better options out there.
Before getting into options, I’m curious about real reliability of these
drives:
1) How often are DC S3700's failing in your deployments?
2) If you have SSD journals at a ratio of 1 to 4 or 5, how painful is the
backfilling which results from an SSD failure? Have you considered tricks like
increasing the down out interval so backfilling doesn’t happen in this case
(leaving time for the SSD to be replaced)?
Beyond the usually 5 partitions deployment, is anyone running a RAID1 or
RAID10 for the journals? If so, are you using the raw block devices or
formatting it and storing the journals as files on the SSD array(s)? Recent
discussions seem to indicate that XFS is just as fast as the block dev, since
these drives are so fast.
Next, I wonder how people with puppet/chef/… are handling the
creation/re-creation of the SSD devices. Are you just wiping and rebuilding all
the dependent OSDs completely when the journal dev fails? I’m not keen on
puppetizing the re-creation of journals for OSDs...
We also have this crazy idea of failing over to a local journal file in case
an SSD fails. In this model, when an SSD fails we’d quickly create a new
journal either on another SSD or on the local OSD filesystem, then restart the
OSDs before backfilling started. Thoughts?
Lastly, I would also consider using 2 of the SSDs in a data pool (with the
other 2 SSDs to hold 20 journals — probably in a RAID1 to avoid backfilling 10
OSDs when an SSD fails). If the 10-1 ratio of SSDs would perform adequately,
that’d give us quite a few SSDs to build a dedicated high-IOPS pool.
I’d also appreciate any other suggestions/experiences which might be relevant.
Thanks!
Dan
-- Dan van der Ster || Data & Storage Services || CERN IT Department --
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