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 <[email protected]> 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 <[email protected]<mailto:[email protected]>> 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 <[email protected]<mailto:[email protected]>> 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 <[email protected]<mailto:[email protected]>> 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 -- _______________________________________________ ceph-users mailing list [email protected]<mailto:[email protected]> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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