This is for us peeps using Ceph with VMWare. My current favoured solution for consuming Ceph in VMWare is via RBD’s formatted with XFS and exported via NFS to ESXi. This seems to perform better than iSCSI+VMFS which seems to not play nicely with Ceph’s PG contention issues particularly if working with thin provisioned VMDK’s. I’ve still been noticing some performance issues however, mainly noticeable when doing any form of storage migrations. This is largely due to the way vSphere transfers VM’s in 64KB IO’s at a QD of 32. vSphere does this so Arrays with QOS can balance the IO easier than if larger IO’s were submitted. However Ceph’s PG locking means that only one or two of these IO’s can happen at a time, seriously lowering throughput. Typically you won’t be able to push more than 20-25MB/s during a storage migration There is also another issue in that the IO needed for the XFS journal on the RBD, can cause contention and effectively also means every NFS write IO sends 2 down to Ceph. This can have an impact on latency as well. Due to possible PG contention caused by the XFS journal updates when multiple IO’s are in flight, you normally end up making more and more RBD’s to try and spread the load. This normally means you end up having to do storage migrations…..you can see where I’m getting at here. I’ve been thinking for a while that CephFS works around a lot of these limitations. 1. It supports fancy striping, so should mean there is less per object contention 2. There is no FS in the middle to maintain a journal and other associated IO 3. A single large NFS mount should have none of the disadvantages seen with a single RBD 4. No need to migrate VM’s about because of #3 5. No need to fstrim after deleting VM’s 6. Potential to do away with pacemaker and use LVS to do active/active NFS as ESXi does its own locking with files With this in mind I exported a CephFS mount via NFS and then mounted it to an ESXi host as a test. Initial results are looking very good. I’m seeing storage migrations to the NFS mount going at over 200MB/s, which equates to several thousand IO’s and seems to be writing at the intended QD32. I need to do more testing to make sure everything works as intended, but like I say, promising initial results. Further testing needs to be done to see what sort of MDS performance is required, I would imagine that since we are mainly dealing with large files, it might not be that critical. I also need to consider the stability of CephFS, RBD is relatively simple and is in use by a large proportion of the Ceph community. CephFS is a lot easier to “upset”. Nick |
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