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# Using Partitions
[[IRC]], #hurd, 2007-07-04.
<azeem-uni> so, is there a way to use a Debian GNU/Hurd partition
(/dev/hda6) with qemu directly?
<tschwinge> Don't dare to do that, please.
<tschwinge> It will lead to inconsistencies.
<tschwinge> Because the Linux kernel thinks that it has complete control
over the disk, or something.
<tschwinge> In theory you could run something like ``-hda /dev/hda'',
having GRUB installed on there to offer you to boot your Hurd system from
hda6 and that will even work, but then don't get the idea to stop qemu,
mount that partition on your Linux system and restart qemu. That's where
I got lots of inconsistencies then, afterwards.
<azeem-uni> it's probably the same problem as having that partition
mounted, suspending to disk, booting into it in the Hurd, and resume
Linux
<neal> right
<tschwinge> That's a different problem.
<tschwinge> Then the partitoon is still mounted.
<neal> no, I think it is basically the same problem
<tschwinge> The file system stuff is cached in the kernel.
<neal> you have data that has not been written to disk yet
<tschwinge> Right.
<neal> and neither is prepared for the resource to be shared
<tschwinge> In the azeem-uni scenarion the data is on the file system layer
and in my scenarion it's some disk block caching inside the Linux kernel,
I guess.
<azeem-uni> anyway, do you guys think if I use -hda /dev/hda and tell Grub
to boot off /dev/hda6, that the rest of hda should be fine, right?
<azeem-uni> maybe adding -snapshot makes it totally safe
<neal> azeem: Should be fine.
<tschwinge> Yes.
The problem is actually that the linux block cache doesn't make any consistency
between /dev/hda and /dev/hda6, so if you give /dev/hda to qemu, qemu writings
won't be consistent with mounting /dev/hda6 in linux. You can give /dev/hda6
directly to qemu and it will be fine.
# Host-side Writeback Caching
IRC, freenode, #hurd, 2011-06-07
<braunr> hm, i guess i should have used cache=writeback with kvm before
starting the debian installer :/
<braunr> ah yes, much better
<braunr> this shows how poor the state of our I/O drivers and subsystem is
:/
<antrik> indeed... still no clustered pageout :-(
<braunr> and no I/O scheduler either
<braunr> although an I/O scheduler has limited value without clustered
pageouts
<braunr> since one of its goals is to pack related I/O requests together eh
<braunr> i wonder if the wiki mentions using cache=writeback to speed up
qemu performances
<braunr> it would help those unable to use kvm a lot
<braunr> and even those running kvm too
<braunr> kvm -m $RAM \ -monitor stdio \ -drive
cache=writeback,index=0,media=disk,file=hd0.img \
<braunr> etc..
<braunr> the idea is that qemu doesn't open its disk file synchronously
<braunr> changes are queued in the host page cache before being flushed to
the disk image
<braunr> but if you brutally close your qemu instance, you're likely to
loose file system consistency
<braunr> ext2fs will think it has committed its metadata to the disk, but
the disk image won't be updated synchronously
<braunr> on my machine (which is quite fast), my kvm has installed debian
like 10 times faster than without the option
<antrik> braunr: I don't think killing qemu should hurt in this
case... probably only matters when the host machine dies
<braunr> antrik: ah yes, right
<braunr> it really makes everything faster, even downloading, since I/O
requests aren't interleaved between networking RPCs
<antrik> regarding I/O sheduler... this discussion came up before, but I
don't remember the outcome -- doesn't the glued Linux driver actually
come with one?
<braunr> i don't remember either
<antrik> braunr: err... I don't think interleaving has anything to do with
it... I guess it's simply the fact that downloading writes the result to
disk, which suffers from lacking clustered pageout like everything else
<antrik> (my internet connection is too slow though to notice :-) )
<braunr> well, if there is no I/O during downloading, downloading is faster
:)
IRC, freenode, #hurd, 2011-06-08
<braunr> youpi: does xen provide disk caching options ?
<youpi> through a blktap, probably
<braunr> ok
([[microkernel/mach/gnumach/ports/Xen]], *Host-side Writeback Caching*.)
<braunr> we should find the pages mentioning qemu on the wiki and add the
options to enable disk image caching
<braunr> it really makes the hurd run a lot faster
<braunr> as a workaround for emulators until I/O is reworked, ofc
IRC, freenode, #hurd, 2011-06-09
<gnu_srs> braunr recommends to use writeback caching with kvm. Is this
really recommended with the frequent crashes I experience?
<youpi> provided that you terminate your kvm normaly (i.e. quitting it, not
killing it), there should be no difference
<jkoenig> I think the host's stability is what matters
<jkoenig> the data presumably sits in linux's cache even if qemu dies
violently
<gnu_srs> But the freezes I see force me to kill kvm :-(
<youpi> maybe kvm doesn't even do caching indeed, I don't know
<youpi> gnu_srs: you can quit even when frozen
<youpi> use the console
<youpi> (the kvm console)
<jkoenig> gnu_srs, "Writeback caching will report data writes as completed
as soon as the data is present in the host page cache. This is safe as
long as you trust your host. If your host crashes or loses power, then
the guest may experience data corruption." (from the qemu manpage)
IRC, freenode, #hurd, 2011-06-11
<gnu_srs> braunr: If you are online. For me setting the parameters -drive
cache=writeback,index=0,media=disk,file=hd0.img does not show any speed
improvement at all compared to the default.
<braunr> gnu_srs: what's your complete qemu command line ?
<gnu_srs> kvm -m 1024 -net nic,model=rtl8139 -net
user,hostfwd=tcp::5556-:22 -drive
cache=writeback,index=0,media=disk,file=hd0.img -cdrom netinst.iso
<braunr> what qemu version ?
<gnu_srs> qemu-kvm 0.14.1+dfsg-1: Sorry, I cannot be online until
tomorrow again.
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