Hi, My guess ( and i did not follow the complete thread) is that for sync io you would need to do the following: - go with mirror if possible. - add a log device for you zil, that will help a lot.
Seb, On Tuesday, April 28, 2015, Gavin Ang <[email protected]> wrote: > Hi, > > Thanks for all the inputs on this issue. I have limited hardware > options cause I am running this on a blade server which only has those 4 > slots for disks. Am using this in production too, so having redundant disks > means I have very little manouvering room. > > > > Anyway, what I was trying to get at was the disparity between host and > guest io performance. Other posts like > http://serverfault.com/questions/425607/kvm-guest-io-is-much-slower-than-host-io-is-that-normal > point to qemu tuning of the disk caching mechanism. There are a lot more > tuning suggestions by IBM at > http://www-01.ibm.com/support/knowledgecenter/linuxonibm/liaat/liaatbpkvmguestcache.htm > so I think there is more I can do before resorting to disk changes. Also, > there is a IBM benchmark presentation at a linux conference (can’t find it > as I write this) which suggests that the kvm overhead for io can be less > than 15% of bare metal performance. > > > > Seems like there is little documentation on how to tweak SmartOS > virtualization. I can see the place I would like to change in the smartvm > script, I just don’t know how to get it done because it seems like that > script is being generated when I issue the vmadm start $uuid command. > > > > The guest machine I am trying to set up is for a database anyway, so > would be mostly sync writes, and looking at the literature out there, it > just seems that there must be a way to narrow the gap between the host and > the guest cause other OS can do it. I should be able to do it for smartOS > too. Question is HOW? > > > > Rgds, > > Gavin > > > > *From:* smartos-discuss [mailto:[email protected] > <javascript:_e(%7B%7D,'cvml','[email protected]');>] > > *Sent:* Tuesday, 28 April, 2015 4:31 PM > *To:* [email protected] > <javascript:_e(%7B%7D,'cvml','[email protected]');> > *Subject:* Digest for smartos-discuss > > > > > > > > > > *Re: [smartos-discuss] Disk IO optimization for Guest OS > <https://www.listbox.com/member/archive/184463/2015/04/20150428042222:AF48A096-ED7F-11E4-8929-BCC322AA7471> > * > > *Sent by Michel Jansens <[email protected] > <javascript:_e(%7B%7D,'cvml','[email protected]');>>* at Tue, 28 > Apr 2015 10:25:55 +0200 > > Hi, Like Greg and Ian said, this slowdown is due to the lack of log > device. There are 2 kinds of writes: -async (asynchronous) writes calls > gets buffered and the write call returns without waiting for the actual > write to disk to complete. This is used to add performance for unimportant > data (e.g.: log files,..) that could be missing on a panic or powerloss. > -sync (synchronous) writes calls wait for the confirmation that data is on > permanent storage before returning. This is used especially in databases > and filesystem internal to ensure coherence of data. You could disable the > sync writes with: zfs set sync=disabled zones/<uuid>-disk0 This will speed > your sync writes quite dramatically. Downside is you could end up with > corrupted data or filesystem if the Host system goes down unexpectedly. You > could limit damage if you snapshot regularly. Snapshots are coherent, so if > you rollback to the latest snapshot you’re safe. I’ve done just this with > some Windows 7 VMs running stuff with non critical data. The speedup is > quite dramatic for IO bound workloads. Another way around this I think, is > to use the recently released LX brand zones where you run Linux natively in > SmartOS (with translated system calls). From the release notes, I think > they implement async writes. That being said, if you use a database with > sync writes that is important to you, you better invest in a pair of small > and fast ssds (log devices) to speed-up the sync writes. — Michel On KVM > zones only does sync writes. 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