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]]
Sent: Tuesday, 28 April, 2015 4:31 PM
To: [email protected] <mailto:[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 < <mailto:[email protected]>
[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. [ trailing quoted section removed ]
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