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I am running a PVE 2.0 on a decent Hardware (2 SATA HDDs as RAID1, 12GB RAM, i7 CPU) but the I/O Performance is very poor inside the VM (Ubuntu 11.10 Server). The very same VM was copied to another Server running simply Ubuntu Server with KVM and had better I/O Perf.

this is how the HDD is shown in the Guest:

ata1: SATA link up 6.0 Gbps (SStatus 133 SControl 300)
ata1.00: ATA-8: ST3000DM001-9YN166, CC49, max UDMA/133
ata1.00: 5860533168 sectors, multi 16: LBA48 NCQ (depth 31/32), AA
ata1.00: configured for UDMA/133
scsi 0:0:0:0: Direct-Access     ATA      ST3000DM001-9YN1 CC49 PQ: 0 ANSI: 5
sd 0:0:0:0: [sda] 5860533168 512-byte logical blocks: (3.00 TB/2.72 TiB)
sd 0:0:0:0: [sda] 4096-byte physical blocks
sd 0:0:0:0: [sda] Write Protect is off
sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00
sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA

I tested with DD:

$ dd bs=1M count=128 if=/dev/zero of=test conv=fdatasync
128+0 records in
128+0 records out
134217728 bytes (134 MB) copied, 19.2222 s, 7.0 MB/s

on the Host, this same Test will result with 156 MB/s in average.

PS: I am using VirtIO and see no error in dmesg.

share|improve this question
when using virtio you should be seeing vda, not sda iirc. – dyasny Apr 17 '12 at 6:56

Try to add both virtio and ide drives to the same VM and compare its speed.

Then, you're use 7200RPM disk optimized for desktop (mostly single thread) use, and it may impact you numbers (in fact Proxmox works with the same disks at the same time as VM). What kind of RAID hardware controller you're using? By the way, most controllers are equipped with cache memory of 256 or 512 Mb these days, and 134 Mb of your test file may be too small site to have clear results (in case you're compare results VM vs raw hardware).

Next, try to upgrade firmware for the drive(s), quite a chance it'll do better after that.

share|improve this answer
ok - thanks, will try the IDE drive to compare. the VM is the only VM running on the server - so no load issues. There isn't a hardware RAID controller, just linux software RAID. – sorsenne Apr 19 '12 at 8:28
Proxmox team recommends not to use software RAID. As always, it looks like 'run VMs on cheap hardware results in extra burden compared to simple run OS on that hardware', so you should upgrade hardware or prepare to see more failures – Alexander Apr 19 '12 at 12:15

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