qcow2 performance has been historically bad regardless of the underlying storage (it is an absolutely terrible storage format), which is why most OpenStack Kilo and later installations instead usually use managed LVM and present LVM volumes as iSCSI volumes to QEMU, because using raw LVM volumes directly works quite a bit better (especially since you can do "thick" volumes, which get you the best performance, without having to zero out a large file on disk). But Cloudstack doesn't use that paradigm. Still, you can get much better performance with qcow2 regardless:
1) Create a disk offering that creates 'sparse' qcow2 volumes (the 'sparse' provisioning type). Otherwise every write is actually multiple writes -- one to extend the previous qcow2 file, one to update the inode with the new file size, and one to update the qcow2 file's own notion of how long it is and what all of its sections are, and one to write the actual data. And these are all *small* random writes, which SSD's have historically been bad at due to write zones. Note that if you look at a freshly provisioned 'sparse' file in the actual data store, it might look like it's taking up 2tb of space, but it's actually taking up only a few blocks. 2) In that disk offering, if you care more about performance than about reliability, set the caching mode to 'writeback'. (The default is 'none'). This will result in larger writes to the SSD, which it'll do at higher rates of speeds than small writes. The downside is that your hardware and OS better be *ultra* reliable with battery backup and clean shutdown in case of power failure and etc., or the data in question is toast if something crashes or the power goes out. So consider how important the data is before selecting this option. 3) If you have a lot of time and want to pre-provision your disks in full, in that disk offering set the provisioning type to 'fat'. This will pre-zero a qcow2 file of the full size that you selected. Be aware that Cloudstack does this zeroing of a volume commissioned with this offering type *when you attach it to a virtual machine*, not when you create it. So attach it to a "trash" virtual machine first before you attach it to your "real" virtual machine, unless you want a lot of downtime waiting for it to zero. But assuming you have a host filesystem that properly allocates files on a per-extent basis, and the extents match up with the underlying SSD write block size well, you should be able to get within 5% of hardware performance with 'fat' qcow2. (With 'thin' you can still come within 10% of that, which is why 'thin' might be the best for most workloads that require performance, and 'thin' doesn't waste space on blocks that have never been written and doesn't tie up your storage system for hours zeroing out a 2tb qcow2 file, so consider that if thinking 'fat'). 4) USE XFS AS THE HOST FILESYSTEM FOR THE DATASTORE. ext4 will be *terrible*. I'm not sure what causes the bad will between ext4 on the storage host and qcow2, but I've seen it multiple times in my own testing of raw libvirt (no CloudStack). As for btrfs, btrfs will be terrible with regular 'thin' qcow2. There is an interaction between its write cycles and qcow2's write patterns that, as with ext4, causes very slow performance. I have not tested sparse qcow2 with btrfs because I don't trust btrfs, it has many design decisions reminiscent of ReiserFS, which ate many Linux filesystems back during the day. I have not tested ZFS. The ZFS on Linux implementation generally has good but not great performance, it was written for reliability, not performance, so it seemed a waste of my time to test it. I may do that this weekend however just to see. I inherited a PCIe M2.e SSD, you see, and want to see what having that as the write cache device will do for performance.... 5) For the guest filesystem it really depends on your workload and the guest OS. I love ext4 for reliability inside a virtual machine, because you can't just lose an entire ext4 filesystem (it's based on ext2/ext3, which in turn were created when hardware was much less reliable than today and thus has a lot of features to keep you from losing an entire filesystem just because a few blocks went AWOL), but it's not a very fast filesystem. Xfs in my testing has the best performance for virtually all workloads. Generally, I use ext4 for root volumes, and make decisions for data volumes based upon how important the performance versus reliability equation works out for me. I have a lot of ext4 filesystems hanging around for data that basically sits there in place without many writes but which I don't want to lose. For best performance of all, manage this SSD storage *outside* of Cloudstack as a bunch of LVM volumes which are exported to virtual machine guests via LIO (iSCSI). Even 'sparse' LVM volumes perform better than qcow2 'thin' volumes. If you choose to do that, there's some LIO settings that'll make things faster for a write-heavy load. But you'll be managing your volumes manually yourself rather than having Cloudstack do it. Which is OK if you're provisioning a database server under Cloudstack and don't intend on offering this expensive SSD storage to all customers, but obviously doesn't scale to ISP public cloud levels. For that you'll need to figure out how to integrate Cloudstack with something like Cinder which can do this exporting in an automated fashion. > On Aug 5, 2017, at 09:29, Rodrigo Baldasso <rodr...@loophost.com.br> wrote: > > Yes.. mounting an lvm volume inside the host works great, ~500Mb/s write > speed.. inside the guest i'm using ext4 but the speed is aroung 30mb/s. > > - - - - - - - - - - - - - - - - - - - > > Rodrigo Baldasso - LHOST > > (51) 9 8419-9861 > - - - - - - - - - - - - - - - - - - - > On 05/08/2017 13:26:00, Ivan Kudryavtsev <kudryavtsev...@bw-sw.com> wrote: > Rodrigo, is your fio testing shows great results? What filesystem you are > using? KVM is known to work very bad over BTRFS. > > 5 авг. 2017 г. 23:16 пользователь "Rodrigo Baldasso" > rodr...@loophost.com.br> написал: > > Hi Ivan, > > In fact i'm testing using local storage.. but on NFS I was getting similar > results also. > > Thanks! > > - - - - - - - - - - - - - - - - - - - > > Rodrigo Baldasso - LHOST > > (51) 9 8419-9861 > - - - - - - - - - - - - - - - - - - - > On 05/08/2017 13:03:24, Ivan Kudryavtsev wrote: > Hi, Rodrigo. It looks strange. Check your NFSconfiguration and network > errors, loss. It should work great. > > 5 авг. 2017 г. 22:22 пользователь "Rodrigo Baldasso" > rodr...@loophost.com.br> написал: > > Hi everyone, > > I'm having trouble to archive a good I/O rate using cloudstack qcow2 with > any type of caching (or even disabled). > > We have some RAID-5e SSD arrays which give us a very good rates directly on > the node/host, but on the guest the speed is terrible. > > Does anyone knows a solution/workaround for this? I never used qcow (only > raw+lvm) so I don't know much to do to solve this. > > Thanks!
