Slow KVM HDD Read Speeds, Suggestions?

[o.ii.o]

Hi All, I have an issue I can't seem to really solve at present and that is that disk speeds inside the KVMs are really slow.

4 disk Raid10 array on a 3Ware/LSI 9750 controller, Sata III HDDs.
Disk Read on Host Node: 1073741824 bytes (1.1 GB) copied, 1.08169 s, 993 MB/s
Disk Read inside 8gb KVM: 1073741824 bytes (1.1 GB) copied, 3.79095 s, 283 MB/s

Now I also have an SSD on the node, so I even tried fooling around with Flashcache and the same results, the actual disk reads won't increase. They drop as low as 180MB/s, it's like it literally refuses to go beyond 300MB/s. Cache for the KVMs is set to "No Cache."

Any pointers as to what is limiting the disk reads inside KVM would be appreciated. HDD mode is Virtio/Raw.

 

[mir]

4 disk Raid10 array on a 3Ware/LSI 9750 controller, Sata III HDDs.
Cache for the KVMs is set to "No Cache."

Any pointers as to what is limiting the disk reads inside KVM would be appreciated. HDD mode is Virtio/Raw.

If you have a RAID controller which is batteri backup then try enable "write through".

 

[dietmar]

Disk Read on Host Node: 1073741824 bytes (1.1 GB) copied, 1.08169 s, 993 MB/s

That value is a bit high. What benchmark do you use?

 

[Nemesiz]

I`d like to know too.

Another example

# dd if=/dev/zero of=out bs=128k count=10k
10240+0 records in
10240+0 records out
1342177280 bytes (1.3 GB) copied, 0.267456 s, 5.0 GB/s