Mailinglist Archive: opensuse (1468 mails)

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Re: [opensuse] [OT] is there a virtual machine that IMPROVE performance by using harddisk as harddisk image
  • From: "Boyd Stephen Smith Jr." <bss@xxxxxxxxxxxxxxxxx>
  • Date: Tue, 2 Jun 2009 10:57:46 -0500
  • Message-id: <200906021057.51037.bss@xxxxxxxxxxxxxxxxx>
In <4A2547B0.8020706@xxxxxxxxx>, Sandy Drobic wrote:
Lars Marowsky-Bree wrote:
On 2009-06-02T22:07:17, Zhang Weiwu <zhangweiwu@xxxxxxxxxx> wrote:
As far as I know if the hard disk of the guest os is an image file on
the host OS, the host OS treats it as a file and offer to cache it in
the main memory. However if vmplayer access a partition or harddisk it
is not cached by the host OS's file system. That might make it even
slower. Unless of course the database is in particular on a raw device
that is directly accessed from the guest OS, as Sandy pointed out.

The caching doesn't make a difference, as it is not really different.

The question itself is not relevant. Fact is that performance will
increase a lot if you use a raw device.

Well, then that software is written poorly. For example, the kernel can use
a file or a partition/disk for swap. Both are virtually the same speed.
There's few reasons accessing physical storage through a file would be any
slower that accessing it through a block device. (The most noticable would
be severe fragmentation, but modern filesystems don't suffer from that
often.)

In the rare case that the program knows how to buffer/cache the device
better than the kernel, using the O_DIRECT flag when opening the file/block
device will cause all reads/writes to bypass the kernels buffer/cache
system.
--
Boyd Stephen Smith Jr. ,= ,-_-. =.
bss@xxxxxxxxxxxxxxxxx ((_/)o o(\_))
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