Date: Thu, 27 Aug 2015 09:31:50 +0000 From: Matt Churchyard <matt.churchyard@userve.net> To: Marcus Reid <marcus@blazingdot.com>, Vick Khera <vivek@khera.org> Cc: "freebsd-virtualization@freebsd.org" <freebsd-virtualization@freebsd.org> Subject: RE: Options for zfs inside a VM backed by zfs on the host Message-ID: <1a6745e27d184bb99eca7fdbdc90c8b5@SERVER.ad.usd-group.com> In-Reply-To: <20150827062015.GA10272@blazingdot.com> References: <CALd%2BdcfJ%2BT-f5gk_pim39BSF7nhBqHC3ab7dXgW8fH43VvvhvA@mail.gmail.com> <20150827061044.GA10221@blazingdot.com> <20150827062015.GA10272@blazingdot.com>
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> On Wed, Aug 26, 2015 at 11:10:44PM -0700, Marcus Reid wrote: > On Wed, Aug 26, 2015 at 05:25:52PM -0400, Vick Khera wrote: > > > Opinions? Preferably well-reasoned ones. :) > > >=20 > > However, having the ARC eating up lots of memory twice seems pretty=20 > > bletcherous. You can probably do some tuning to reduce that, but I=20 > > never liked tuning the ARC much. > I just realized that you can turn primarycache off per-dataset. Does it = make more sense to turn primarycache=3Dnone on the zvol on the host, or > o= n the datasets in the vm? I'm thinking on the host, but it might be worth = experimenting. I'd be very wary of disabling ARC on the main host, it can have pretty seri= ous side effects. It could possibly be useful in the guest though, as data = should be cached already by ARC on the host, you're just going through an e= xtra step of reading through the virtual disk driver, and into host ARC, in= stead of directly from the guest memory. Would need testing to know what pe= rformance was like and if there are any side effects. I do agree that it doesn't seem unnecessary to have any redundancy in the g= uest if the host pool is redundant. Save for any glaring bugs in the virtua= l disk emulation, you wouldn't expect to get errors on the guest pool if th= e host pool is already checksumming the data. It's also worth testing with guest ARC enabled but just limited to a fairly= small size, so you're not disabling it entirely, but doing at little doubl= e-caching as possible. ZFS features seems perfect for virtual hosts, although it's not ideal that = you have to give up a big chunk of host RAM for ARC. You may also find that= you need to limit host ARC, then only use "MAX_RAM - MY_ARC_LIMIT" for gue= sts. Otherwise you'll have ZFS and VMs fighting for memory and enough of us= have seen what shouldn't, but unfortunately does happen in that situation. Matt - > Marcus > _______________________________________________ > freebsd-virtualization@freebsd.org mailing list https://lists.freebsd.org= /mailman/listinfo/freebsd-virtualization > To unsubscribe, send any mail to "freebsd-virtualization-unsubscribe@free= bsd.org"
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