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[Xen-users] Terribad "file:" performance (vs reasonable "phy:" perf)


I've done a bit more research, and found some answers to my earlier question 
about qemu-dm.  It would seem that a 64-bit 3.x kernel does not have blktap 
drivers available.  And, according to the docs in 
docs/misc/xl-disk-configuration.txt, when a domU disk is specified as:


the rule seems to be: "For block device type 'file' with format raw or when no 
format specfied, tapdisk2 is used when present otherwise qemu fallback option 
is used."

I dug through some older sources that suggested using the "phy:" 
block-device-type with a loop device (e.g., /dev/loop0).  This worked, and gave 
an order-of-magnitude improvement.  Here's what I've observed: on a quad-core 
i5-2400 with a single sata HD, I see this type of performance:

  * A physical disk partition           - phy:/dev/sda7       - ~55 MB/s
  * A loop device using the phy: driver - phy:/dev/loop0      - ~35 MB/s
  * A qemu-dm fallback                  - file:/xen/domU.img  - < 3 MB/s (!)

Before pursuing this any further, it would be reassuring to hear that other 
people have seen similar results, or if I'm barking up the wrong tree.  
Specifically, my configuration is Xen-4.1.2 on a 64-bit Linux-3.1.0 dom0 
(dom0_mem=2048), with a 64-bit Linux-3.1.0 domU configured as a PV host.  
Neither host nor guest is multilib or has 32-bit libraries.  And, I'm using xl 
(not xm) to start my instance.  Have the collective "you" seen numbers like 
these, or is this a sign that I'm missing something?

In addition, even though that same doc suggests that these "device type 
specifiers" like "file:" or "phy:" are deprecated, until a blktap driver is 
ported to a pure-64-bit environment, they are still necessary, right?  My 
64-bit build doesn't have blktapctl (or whatever that daemon is called), and 
when I leave out the type specifier, the domU hangs on waiting for XENBUS, 
which--along with the various log outputs--seemed to indicate that it was stuck 
trying to load a blktap driver which doesn't exist.

In short, I'd like to have a reasonable assurance that my choice for non-fixed 
partitions (along with my other system constraints) is using a "phy:" device 
type with either: 1) loop devices I create by hand at the start of each VM, or 
2) using LVM, which creates virtual block devices.

Are those my choices aside from qemu-dm (which appears to suck wind)?


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