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From Rafael Weingärtner <rafaelweingart...@gmail.com>
Subject Re: Caching modes
Date Tue, 20 Feb 2018 13:03:53 GMT
I have no idea how it can change the performance. If you look at the
content of the commit you provided, it is only the commit that enabled the
use of getCacheMode from disk offerings. However, it is not exposing any
way to users to change that value/configuration in the database. I might
have missed it; do you see any API methods that receive the parameter
"cacheMode" and then pass this parameter to a "diskOffering" object, and
then persist/update this object in the database?

May I ask how are you guys changing the cacheMode configuration?

On Tue, Feb 20, 2018 at 9:56 AM, Paul Angus <paul.angus@shapeblue.com>
wrote:

> I'm working with some guys who are experimenting with the setting as if
> definitely seems to change the performance of data disks.  It also changes
> the XML of the VM which is created.
>
> p.s.
> I've found this commit;
>
> https://github.com/apache/cloudstack/commit/1edaa36cc68e845a42339d5f267d49
> c82343aefb
>
> so I've got something to investigate now, but API documentation must
> definitely be askew.
>
>
>
> paul.angus@shapeblue.com
> www.shapeblue.com
> 53 Chandos Place, Covent Garden, London  WC2N 4HSUK
> @shapeblue
>
>
>
>
> -----Original Message-----
> From: Rafael Weingärtner [mailto:rafaelweingartner@gmail.com]
> Sent: 20 February 2018 12:31
> To: dev <dev@cloudstack.apache.org>
> Subject: Re: Caching modes
>
> 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
>



-- 
Rafael Weingärtner

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