Re: [Xen-users] IO is a big difference between the "file" and "phy"

[Florian Heigl <florian.heigl@xxxxxxxxx>]

Hi,

> Do you know what direct I/O is?
> IIRC by default dd does not use direct or sync, so your test will be buffered.

for the DD testing add conv=fdatasync to get real numbers.
i.e.
dd if=/dev/zero of=mytestfile bs=1024k count=1024 conv=fdatasync


File: uses the linux loop driver.
Many people have lost data in that mode, because of write-caching in
the host, i.e. if the dom0 crashes or something, you're likely to have
corrupted virtual machines.
Also the loop device is f*****ing slow, it just sometimes appears
faster b/c of the caching. (NetBSD had a much better loop device)

If you want fs based storage, switch over to tap:aio or what it's called.
A more modern option is the LVMTHIN target for device mapper, but that
requires some good LVM knowledge.

Florian

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