Re: Shouldn't cache=none be the default for drives?
08.04.2010 09:07, Thomas Mueller wrote: [] This helped alot: I enabled deadline block scheduler instead of the default cfq on the host system. tested with: Host Debian with scheduler deadline, Guest Win2008 with Virtio and cache=none. (26MB/s to 50MB/s boost measured) Maybe this is also true for Linux/Linux. I expect that scheduler noop for linux guests would be good. Hmm. I wonder why it helped. In theory, host scheduler should not change anything for cache=none case, at least for raw partitions of LVM volumes. This is because with cache=none, the virtual disk image is opened with O_DIRECT flag, which means all I/O bypasses host scheduler and buffer cache. I tried a few quick tests here, -- with LVM volumes it makes no measurable difference. But if the guest disk images are on plain files (also raw), scheduler makes some difference, and indeed deadline works better. Maybe you were testing with plain files instead of block devices? Thanks! /mjt -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Shouldn't cache=none be the default for drives?
Am Thu, 08 Apr 2010 10:05:09 +0400 schrieb Michael Tokarev: 08.04.2010 09:07, Thomas Mueller wrote: [] This helped alot: I enabled deadline block scheduler instead of the default cfq on the host system. tested with: Host Debian with scheduler deadline, Guest Win2008 with Virtio and cache=none. (26MB/s to 50MB/s boost measured) Maybe this is also true for Linux/Linux. I expect that scheduler noop for linux guests would be good. Hmm. I wonder why it helped. In theory, host scheduler should not change anything for cache=none case, at least for raw partitions of LVM volumes. This is because with cache=none, the virtual disk image is opened with O_DIRECT flag, which means all I/O bypasses host scheduler and buffer cache. I tried a few quick tests here, -- with LVM volumes it makes no measurable difference. But if the guest disk images are on plain files (also raw), scheduler makes some difference, and indeed deadline works better. Maybe you were testing with plain files instead of block devices? ah yes, qcow2 images. - Thomas -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Shouldn't cache=none be the default for drives?
Am Thu, 08 Apr 2010 06:09:05 + schrieb Thomas Mueller: Am Thu, 08 Apr 2010 10:05:09 +0400 schrieb Michael Tokarev: 08.04.2010 09:07, Thomas Mueller wrote: [] This helped alot: I enabled deadline block scheduler instead of the default cfq on the host system. tested with: Host Debian with scheduler deadline, Guest Win2008 with Virtio and cache=none. (26MB/s to 50MB/s boost measured) Maybe this is also true for Linux/Linux. I expect that scheduler noop for linux guests would be good. Hmm. I wonder why it helped. In theory, host scheduler should not change anything for cache=none case, at least for raw partitions of LVM volumes. This is because with cache=none, the virtual disk image is opened with O_DIRECT flag, which means all I/O bypasses host scheduler and buffer cache. I tried a few quick tests here, -- with LVM volumes it makes no measurable difference. But if the guest disk images are on plain files (also raw), scheduler makes some difference, and indeed deadline works better. Maybe you were testing with plain files instead of block devices? ah yes, qcow2 images. ... but does the scheduler really now about O_DIRECT? isn't O_DIRECT meant to bypass only buffers (aka return write not before it really hit the disk)? my understanding is that the scheduler is layer down the stack. but only guessing - i'm not a kernel hacker. :) - Thomas -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Shouldn't cache=none be the default for drives?
On Thu, Apr 08, 2010 at 10:05:09AM +0400, Michael Tokarev wrote: LVM volumes. This is because with cache=none, the virtual disk image is opened with O_DIRECT flag, which means all I/O bypasses host scheduler and buffer cache. O_DIRECT does not bypass the I/O scheduler, only the page cache. -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Shouldn't cache=none be the default for drives?
Hello, I'm conducting some performancetests with KVM-virtualized CentOSes. One thing I noticed is that guest I/O performance seems to be significantly better for virtio-based block devices (drives) if the cache=none argument is used. (This was with a rather powerful storage system backend which is hard to saturate.) So: Why isn't cache=none be the default for drives? -- Troels -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Shouldn't cache=none be the default for drives?
Troels Arvin wrote: Hello, I'm conducting some performancetests with KVM-virtualized CentOSes. One thing I noticed is that guest I/O performance seems to be significantly better for virtio-based block devices (drives) if the cache=none argument is used. (This was with a rather powerful storage system backend which is hard to saturate.) So: Why isn't cache=none be the default for drives? Is that the right question? Or is the right question Why is cache=none faster? What did you use for measuring the performance? I have found in the past that virtio block device was slower than IDE block device emulation. Gordan -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Shouldn't cache=none be the default for drives?
Am Wed, 07 Apr 2010 16:39:41 +0200 schrieb Troels Arvin: Hello, I'm conducting some performancetests with KVM-virtualized CentOSes. One thing I noticed is that guest I/O performance seems to be significantly better for virtio-based block devices (drives) if the cache=none argument is used. (This was with a rather powerful storage system backend which is hard to saturate.) So: Why isn't cache=none be the default for drives? while ago i suffered poor performance of virtio and win2008. This helped alot: I enabled deadline block scheduler instead of the default cfq on the host system. tested with: Host Debian with scheduler deadline, Guest Win2008 with Virtio and cache=none. (26MB/s to 50MB/s boost measured) Maybe this is also true for Linux/Linux. I expect that scheduler noop for linux guests would be good. - Thomas -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
