Re: [CentOS] KVM Host Disk Performance
On Wed, 6 Apr 2011, Scott Robbins wrote: Not all that unique, but a bit better--I think it's VolumeGroup00/lvm_root, VolumeGroup00/lvm_swap, and things like that. (Keeping both LVs in the same VG by default.) As far as I know it's much better than that: The volume group by default with EL6 is vg_$HOSTNAME (with some characters stripped). Even EL5 only created one VG. jh ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
On 5.4.2011 21.49, compdoc wrote: For reasons of speed and ease of maintenance and backups, what I've settled on is: a small separate drive for the host to boot from, a small separate drive for the guest OSes (I like using qcow2 on WD Raptors), and then a large array on a raid controller for storage which the guests and host can share access to. Aren't the guests and host then competing for that large array? How is this arrangement better than other setups, for example having the host each guest (with their associated data) each on their own disk, or partition? - Jussi -- Jussi Hirvi * Green Spot Topeliuksenkatu 15 C * 00250 Helsinki * Finland Tel. +358 9 493 981 * Mobile +358 40 771 2098 (only sms) jussi.hi...@greenspot.fi * http://www.greenspot.fi ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
On Wed, 6 Apr 2011, Jussi Hirvi wrote: On 5.4.2011 21.49, compdoc wrote: For reasons of speed and ease of maintenance and backups, what I've settled on is: a small separate drive for the host to boot from, a small separate drive for the guest OSes (I like using qcow2 on WD Raptors), and then a large array on a raid controller for storage which the guests and host can share access to. Aren't the guests and host then competing for that large array? How is this arrangement better than other setups, for example having the host each guest (with their associated data) each on their own disk, or partition? Doesn't it feel a bit wrong to be tying physical infrastructure to VMs in that way? I'd have thought the large iSCSI device providing storage (not all necessarily identical) to a Virtual Server that then slices and dices appropriately to VMs seems a lot more sane. jh ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
Direct comparisons between the two were difficult to judge, but the general result was that the Host was between 2:1 and 3:1 better than the Guest, which seems to be a rather large performance gap. Latency differences were all over the map, which I find puzzling. The Host is 64-bit and the Guest 32-bit, if that makes any difference. Perhaps caching between Host and Guest accounts for some of the differences. It does sound as if the guests are relying on the host rather than accessing the block device directly. Drives should not use much cpu overhead thanks to DMA and improvements to drivers and hardware. When it's done correctly the host has little work to do. That doesn't sound like what's happening with your setup. Basically, you have to think about the guests as independent systems which are competing for disk access with the other guests, and with the host. If you have just one drive or array that's used by all, that's a large bottleneck. I've been working with VMs for a while now and have tried various ways to set up guests. Block devices can be done with or without LVM, although I've stopped using LVM on my systems these days. For reasons of speed and ease of maintenance and backups, what I've settled on is: a small separate drive for the host to boot from, a small separate drive for the guest OSes (I like using qcow2 on WD Raptors), and then a large array on a raid controller for storage which the guests and host can share access to. ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
On Tue, Apr 5, 2011 at 11:49 AM, compdoc comp...@hotrodpc.com wrote: I've been working with VMs for a while now and have tried various ways to set up guests. Block devices can be done with or without LVM, although I've stopped using LVM on my systems these days. Just curious, why have you stopped using LVM? I've found it to be useful for allocating disk space to to KVM for virtual machines. I usually set up logical volumes on a separate volume group as block devices for the virtual machine to use. If there's an issue with this, I'd like to know about it. -Iain -- -- - Iain Morris iain.t.mor...@gmail.com ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
Just curious, why have you stopped using LVM? Simply for ease of maintenance: some recovery and backup utilities like clonezilla can't work with LVM. And because the same names for volume groups are used for each centos install, so trying to attach a drive or volume to a new system for rescue causes conflicts unless you take steps and use unique names from the start. (Although I hear that newer versions of centos/RH will create unique names for you) As I said, LVM works fine for VMs and can be used slice up a volume for guests to be used as a true block device. By the way, a true block device means a raw partition on the disk is given to the guest to format and use as its own - so no existing file system is present. It's almost like giving a guest its own drive to work from, and should operate at the same native speeds as the host. ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
On Tue, Apr 05, 2011 at 08:22:08PM -0600, compdoc wrote: Just curious, why have you stopped using LVM? Simply for ease of maintenance: some recovery and backup utilities like clonezilla can't work with LVM. And because the same names for volume groups are used for each centos install, so trying to attach a drive or volume to a new system for rescue causes conflicts unless you take steps and use unique names from the start. (Although I hear that newer versions of centos/RH will create unique names for you) Not all that unique, but a bit better--I think it's VolumeGroup00/lvm_root, VolumeGroup00/lvm_swap, and things like that. (Keeping both LVs in the same VG by default.) -- Scott Robbins PGP keyID EB3467D6 ( 1B48 077D 66F6 9DB0 FDC2 A409 FA54 EB34 67D6 ) gpg --keyserver pgp.mit.edu --recv-keys EB3467D6 Xander: It's time for me to act like a man... and hide. ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
On 04/04/2011 09:00 AM, compdoc wrote: It's possible to set up guests to use a block device that will get you the same disk I/O as the underlying storage. Is that what you're seeing? What speed does the host see when benchmarking the RAID volumes, and what speeds do the guests see? Yes, I have been going on the assumption that I get close to native block device performance, but the test results tell me otherwise. I see array rebuild data rates which seem reasonable ... in the order of 60 to 80 MBytes/sec. I'm using 256k chunks, with the stride size set to match the number of data drives. Using bonnie++, I mounted one of the Guest RAID-6 filesystems on the Host, ran the default tests, unmounted, then booted the Guest and ran the same default tests. The amount of RAM assigned was the same for both, to level the playing field a bit. Direct comparisons between the two were difficult to judge, but the general result was that the Host was between 2:1 and 3:1 better than the Guest, which seems to be a rather large performance gap. Latency differences were all over the map, which I find puzzling. The Host is 64-bit and the Guest 32-bit, if that makes any difference. Perhaps caching between Host and Guest accounts for some of the differences. At the moment my questions tend to be a bit academic. I'm primarily wondering if RAID-10 is paranoid enough given the current quality of WD CaviarBlack drives (better than dirt-cheap consumer drives, but not enterprise grade). My second question relates to whether or not the added overhead of using something like qcow2 would be offset by the advantages of more space efficiency and the copy-on-write feature. I'd love to hear what other software RAID users think, especially regarding large-capacity drives. It's rare for a modern drive to hand out bad data without an accompanying error condition (which the md driver should handle), but I have read that uncaught bad data is possible and would not be flagged in RAID arrays which don't use parity calculations. Chuck ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
On 04/04/11 11:32 AM, Chuck Munro wrote: I'd love to hear what other software RAID users think, especially regarding large-capacity drives. It's rare for a modern drive to hand out bad data without an accompanying error condition (which the md driver should handle), but I have read that uncaught bad data is possible and would not be flagged in RAID arrays which don't use parity calculations. AFAIK, no standard raid modes verify parity on reads, as this would require reading the whole slice for every random read. Only raid systems like ZFS that use block checksuming can verify data on reads. parity (or mirrors) are verified by doing 'scrubs' Further, even if a raid DID verify parity/mirroring on reads, this would at best create a nonrecoverable error (bad data on one of the N drives in the slice, no way of knowing which one is the bad one). ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
[CentOS] KVM Host Disk Performance
Hello all, I'm having quite an interesting time getting up to speed with KVM/QEMU and the various ways of creating virtual Guest VMs. But disk I/O performance remains a bit of a question mark for me. I'm looking for suggestions and opinions This new machine has tons of disk space, lots of CPU cores and loads of RAM, so those are not issues. I currently have several software RAID-6 md's, each of which is about 700 GBytes. The md's are created as partitionable arrays, and I assign the resulting raw disk devices to each CentOS Guest OS, which then partitions the space according to its needs. Unlike using the more common filesystem-in-a-file method, the use of partitionable MD devices usually results in wasted disk space, but my aim is to provide the best performance possible with robust RAID. Is there a better-performing way of assigning disk space to Guest VMs? What about 'qcow2'? I understand its copy-on-write offers several advantages, but does it perform better than using partitionable md arrays or does the underlying filesystem overhead get in the way? Perhaps the native overhead of RAID-6 dual parity makes the whole question moot. Would the use of RAID-10 with hot spares be paranoid enough? I'm using 2 TByte drives which inherently have much greater chances of data errors, so I must keep data reliability in mind. I don't want to start a religious war over RAID and Guest VM disk methodology, but I have the luxury of trying various disk configurations until I get a reasonable balance between performance and reliability with these large drives. So many questions ... so much to learn. Lots of Googling around is quite useful but can lead to information overload :-) Cheers, Chuck ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
Cannot give you any useful input but I am certainly interested in any progress you make, so keep communicating :D Dawid -Original Message- From: centos-boun...@centos.org [mailto:centos-boun...@centos.org] On Behalf Of Chuck Munro Sent: Sonntag, 3. April 2011 19:47 To: CentOS Maiing List Subject: [CentOS] KVM Host Disk Performance Hello all, I'm having quite an interesting time getting up to speed with KVM/QEMU and the various ways of creating virtual Guest VMs. But disk I/O performance remains a bit of a question mark for me. I'm looking for suggestions and opinions This new machine has tons of disk space, lots of CPU cores and loads of RAM, so those are not issues. I currently have several software RAID-6 md's, each of which is about 700 GBytes. The md's are created as partitionable arrays, and I assign the resulting raw disk devices to each CentOS Guest OS, which then partitions the space according to its needs. Unlike using the more common filesystem-in-a-file method, the use of partitionable MD devices usually results in wasted disk space, but my aim is to provide the best performance possible with robust RAID. Is there a better-performing way of assigning disk space to Guest VMs? What about 'qcow2'? I understand its copy-on-write offers several advantages, but does it perform better than using partitionable md arrays or does the underlying filesystem overhead get in the way? Perhaps the native overhead of RAID-6 dual parity makes the whole question moot. Would the use of RAID-10 with hot spares be paranoid enough? I'm using 2 TByte drives which inherently have much greater chances of data errors, so I must keep data reliability in mind. I don't want to start a religious war over RAID and Guest VM disk methodology, but I have the luxury of trying various disk configurations until I get a reasonable balance between performance and reliability with these large drives. So many questions ... so much to learn. Lots of Googling around is quite useful but can lead to information overload :-) Cheers, Chuck ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
Re: [CentOS] KVM Host Disk Performance
I'm having quite an interesting time getting up to speed with KVM/QEMU and the various ways of creating virtual Guest VMs. But disk I/O performance remains a bit of a question mark for me. I'm looking for suggestions and opinions It's possible to set up guests to use a block device that will get you the same disk I/O as the underlying storage. Is that what you're seeing? What speed does the host see when benchmarking the RAID volumes, and what speeds do the guests see? ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
