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From Rafael Weingärtner <rafaelweingart...@gmail.com>
Subject Re: Caching modes
Date Tue, 20 Feb 2018 12:31:19 GMT
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.panic@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.panic@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.angus@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.angus@shapeblue.com
> >> www.shapeblue.com
> >> 53 Chandos Place, Covent Garden, London  WC2N 4HSUK
> >> @shapeblue
> >>
> >>
> >>
> >>
> >> -----Original Message-----
> >> From: Wido den Hollander [mailto:wido@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.angus@shapeblue.com
> >> > www.shapeblue.com
> >> > 53 Chandos Place, Covent Garden, London  WC2N 4HSUK @shapeblue
> >> >
> >> >
> >> >
> >>
> >
> >
> >
> > --
> >
> > Andrija Panić
> >
>
>
>
> --
>
> Andrija Panić
>



-- 
Rafael Weingärtner

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