This cache mode parameter does not exist in "CreateDiskOfferingCmd" command. I also checked some commits from 2, 3, 4 and 5 years ago, and this parameter was never there. If you check the API in [1], you can see that it is not an expected parameter. Moreover, I do not see any use of "setCacheMode" in the code (in case it is updated by some other method). Interestingly enough, the code uses "getCacheMode".
In summary, it is not a feature, and it does not work. It looks like some leftover from dark ages when people could commit anything and then they would just leave a half implementation there in our code base. [1] https://cloudstack.apache.org/api/apidocs-4.11/apis/createDiskOffering.html On Tue, Feb 20, 2018 at 9:19 AM, Andrija Panic <andrija.pa...@gmail.com> wrote: > I can also assume that "cachemode" as API parameter is not supported, since > when creating data disk offering via GUI also doesn't set it in DB/table. > > CM: create diskoffering name=xxx displaytext=xxx storagetype=shared > disksize=1024 cachemode=writeback > > this also does not set cachemode in table... my guess it's not implemented > in API > > Let me know if I can help with any testing here. > > Cheers > > On 20 February 2018 at 13:09, Andrija Panic <andrija.pa...@gmail.com> > wrote: > > > Hi Paul, > > > > not helping directly answering your question, but here are some > > observations and "warning" if client's are using write-back cache on KVM > > level > > > > > > I have (long time ago) tested performance in 3 combinations (this was not > > really thorough testing but a brief testing with FIO and random IO WRITE) > > > > - just CEPH rbd cache (on KVM side) > > i.e. [client] > > rbd cache = true > > rbd cache writethrough until flush = true > > #(this is default 32MB per volume, afaik > > > > - just KMV write-back cache (had to manually edit disk_offering table to > > activate cache mode, since when creating new disk offering via GUI, the > > disk_offering tables was NOT populated with "write-back" sertting/value > ! ) > > > > - both CEPH and KVM write-back cahce active > > > > My observations were like following, but would be good to actually > confirm > > by someone else: > > > > - same performance with only CEPH caching or with only KVM caching > > - a bit worse performance with both CEPH and KVM caching active (nonsense > > combination, I know...) > > > > > > Please keep in mind, that some ACS functionality, KVM live-migrations on > > shared storage (NFS/CEPH) are NOT supported when you use KVM write-back > > cache, since this is considered "unsafe" migration, more info here: > > https://doc.opensuse.org/documentation/leap/virtualization/ > html/book.virt/ > > cha.cachemodes.html#sec.cache.mode.live.migration > > > > or in short: > > " > > The libvirt management layer includes checks for migration compatibility > > based on several factors. If the guest storage is hosted on a clustered > > file system, is read-only or is marked shareable, then the cache mode is > > ignored when determining if migration can be allowed. Otherwise libvirt > > will not allow migration unless the cache mode is set to none. However, > > this restriction can be overridden with the “unsafe” option to the > > migration APIs, which is also supported by virsh, as for example in > > > > virsh migrate --live --unsafe > > " > > > > Cheers > > Andrija > > > > > > On 20 February 2018 at 11:24, Paul Angus <paul.an...@shapeblue.com> > wrote: > > > >> Hi Wido, > >> > >> This is for KVM (with Ceph backend as it happens), the API documentation > >> is out of sync with UI capabilities, so I'm trying to figure out if we > >> *should* be able to set cacheMode for root disks. It seems to make > quite a > >> difference to performance. > >> > >> > >> > >> paul.an...@shapeblue.com > >> www.shapeblue.com > >> 53 Chandos Place, Covent Garden, London WC2N 4HSUK > >> @shapeblue > >> > >> > >> > >> > >> -----Original Message----- > >> From: Wido den Hollander [mailto:w...@widodh.nl] > >> Sent: 20 February 2018 09:03 > >> To: dev@cloudstack.apache.org > >> Subject: Re: Caching modes > >> > >> > >> > >> On 02/20/2018 09:46 AM, Paul Angus wrote: > >> > Hey guys, > >> > > >> > Can anyone shed any light on write caching in CloudStack? cacheMode > >> is available through the UI for data disks (but not root disks), but not > >> documented as an API option for data or root disks (although is > documented > >> as a response for data disks). > >> > > >> > >> What hypervisor? > >> > >> In case of KVM it's passed down to XML which then passes it to Qemu/KVM > >> which then handles the caching. > >> > >> The implementation varies per hypervisor, so that should be the > question. > >> > >> Wido > >> > >> > >> > #huh? > >> > > >> > thanks > >> > > >> > > >> > > >> > paul.an...@shapeblue.com > >> > www.shapeblue.com > >> > 53 Chandos Place, Covent Garden, London WC2N 4HSUK @shapeblue > >> > > >> > > >> > > >> > > > > > > > > -- > > > > Andrija Panić > > > > > > -- > > Andrija Panić > -- Rafael Weingärtner
