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From: Giangiacomo Mariotti on 12 Jul 2010 01:30
Hi, is it a known problem how much slow is Btrfs with kvm/qemu(meaning
that the image kvm/qemu uses as the hd is on a partition formatted
with Btrfs, not that the fs used by the hd inside the kvm environment
is Btrfs, in fact inside kvm the / partition is formatted with ext3)?
I haven't written down the exact numbers, because I forgot, but while
I was trying to make it work, after I noticed how much longer than
usual it was taking to just install the system, I took a look at iotop
and it was reporting a write speed of the kvm process of approximately
3M/s, while the Btrfs kernel thread had an approximately write speed
of 7K/s! Just formatting the partitions during the debian installation
took minutes. When the actual installation of the distro started I had
to stop it, because it was taking hours! The iotop results made me
think that the problem could be Btrfs, but, to be sure that it wasn't
instead a kvm/qemu problem, I cut/pasted the same virtual hd on an
ext3 fs and started kvm with the same parameters as before. The
installation of debian inside kvm this time went smoothly and fast,
like normally it does. I've been using Btrfs for some time now and
while it has never been a speed champion(and I guess it's not supposed
to be one and I don't even really care that much about it), I've never
had any noticeable performance problem before and it has always been
quite stable. In this test case though, it seems to be doing very bad.

cheers

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Giangiacomo
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From: Justin P. Mattock on 12 Jul 2010 02:00
On 07/11/2010 10:24 PM, Giangiacomo Mariotti wrote:
> Hi, is it a known problem how much slow is Btrfs with kvm/qemu(meaning
> that the image kvm/qemu uses as the hd is on a partition formatted
> with Btrfs, not that the fs used by the hd inside the kvm environment
> is Btrfs, in fact inside kvm the / partition is formatted with ext3)?
> I haven't written down the exact numbers, because I forgot, but while
> I was trying to make it work, after I noticed how much longer than
> usual it was taking to just install the system, I took a look at iotop
> and it was reporting a write speed of the kvm process of approximately
> 3M/s, while the Btrfs kernel thread had an approximately write speed
> of 7K/s! Just formatting the partitions during the debian installation
> took minutes. When the actual installation of the distro started I had
> to stop it, because it was taking hours! The iotop results made me
> think that the problem could be Btrfs, but, to be sure that it wasn't
> instead a kvm/qemu problem, I cut/pasted the same virtual hd on an
> ext3 fs and started kvm with the same parameters as before. The
> installation of debian inside kvm this time went smoothly and fast,
> like normally it does. I've been using Btrfs for some time now and
> while it has never been a speed champion(and I guess it's not supposed
> to be one and I don't even really care that much about it), I've never
> had any noticeable performance problem before and it has always been
> quite stable. In this test case though, it seems to be doing very bad.
>
> cheers
>

not sure with butter filesystems.. but, what is the last good kernel?
are you able to bisect?

Justin P. Mattock
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From: Michael Tokarev on 12 Jul 2010 03:10
12.07.2010 09:24, Giangiacomo Mariotti wrote:
> Hi, is it a known problem how much slow is Btrfs with kvm/qemu(meaning
> that the image kvm/qemu uses as the hd is on a partition formatted
> with Btrfs, not that the fs used by the hd inside the kvm environment
> is Btrfs, in fact inside kvm the / partition is formatted with ext3)?
> I haven't written down the exact numbers, because I forgot, but while
> I was trying to make it work, after I noticed how much longer than
> usual it was taking to just install the system, I took a look at iotop
> and it was reporting a write speed of the kvm process of approximately
> 3M/s, while the Btrfs kernel thread had an approximately write speed
> of 7K/s! Just formatting the partitions during the debian installation
> took minutes. When the actual installation of the distro started I had
> to stop it, because it was taking hours! The iotop results made me
> think that the problem could be Btrfs, but, to be sure that it wasn't
> instead a kvm/qemu problem, I cut/pasted the same virtual hd on an
> ext3 fs and started kvm with the same parameters as before. The
> installation of debian inside kvm this time went smoothly and fast,
> like normally it does. I've been using Btrfs for some time now and
> while it has never been a speed champion(and I guess it's not supposed
> to be one and I don't even really care that much about it), I've never
> had any noticeable performance problem before and it has always been
> quite stable. In this test case though, it seems to be doing very bad.

This looks quite similar to a problem with ext4 and O_SYNC which I
reported earlier but no one cared to answer (or read?) - there:
http://permalink.gmane.org/gmane.linux.file-systems/42758
(sent to qemu-devel and linux-fsdevel lists - Cc'd too). You can
try a few other options, esp. cache=none and re-writing some guest
files to verify.

/mjt
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From: Justin P. Mattock on 12 Jul 2010 03:20
On 07/12/2010 12:09 AM, Michael Tokarev wrote:
> 12.07.2010 09:24, Giangiacomo Mariotti wrote:
>> Hi, is it a known problem how much slow is Btrfs with kvm/qemu(meaning
>> that the image kvm/qemu uses as the hd is on a partition formatted
>> with Btrfs, not that the fs used by the hd inside the kvm environment
>> is Btrfs, in fact inside kvm the / partition is formatted with ext3)?
>> I haven't written down the exact numbers, because I forgot, but while
>> I was trying to make it work, after I noticed how much longer than
>> usual it was taking to just install the system, I took a look at iotop
>> and it was reporting a write speed of the kvm process of approximately
>> 3M/s, while the Btrfs kernel thread had an approximately write speed
>> of 7K/s! Just formatting the partitions during the debian installation
>> took minutes. When the actual installation of the distro started I had
>> to stop it, because it was taking hours! The iotop results made me
>> think that the problem could be Btrfs, but, to be sure that it wasn't
>> instead a kvm/qemu problem, I cut/pasted the same virtual hd on an
>> ext3 fs and started kvm with the same parameters as before. The
>> installation of debian inside kvm this time went smoothly and fast,
>> like normally it does. I've been using Btrfs for some time now and
>> while it has never been a speed champion(and I guess it's not supposed
>> to be one and I don't even really care that much about it), I've never
>> had any noticeable performance problem before and it has always been
>> quite stable. In this test case though, it seems to be doing very bad.
>
> This looks quite similar to a problem with ext4 and O_SYNC which I
> reported earlier but no one cared to answer (or read?) - there:
> http://permalink.gmane.org/gmane.linux.file-systems/42758
> (sent to qemu-devel and linux-fsdevel lists - Cc'd too). You can
> try a few other options, esp. cache=none and re-writing some guest
> files to verify.
>
> /mjt
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> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo(a)vger.kernel.org
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> Please read the FAQ at http://www.tux.org/lkml/
>

cool a solution... glad to see... no chance at a bisect with this?
(getting this down too a commit or two makes things easier)

Justin P. Mattock
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From: Giangiacomo Mariotti on 12 Jul 2010 09:30
On Mon, Jul 12, 2010 at 9:17 AM, Justin P. Mattock
<justinmattock(a)gmail.com> wrote:
> On 07/12/2010 12:09 AM, Michael Tokarev wrote:
>>
>> This looks quite similar to a problem with ext4 and O_SYNC which I
>> reported earlier but no one cared to answer (or read?) - there:
>> http://permalink.gmane.org/gmane.linux.file-systems/42758
>> (sent to qemu-devel and linux-fsdevel lists - Cc'd too).  You can
>> try a few other options, esp. cache=none and re-writing some guest
>> files to verify.
>>
>> /mjt
>
> cool a solution... glad to see... no chance at a bisect with this?
> (getting this down too a commit or two makes things easier)
>
> Justin P. Mattock
>
I didn't even say what kernel version I was using, sorry! Kernel
2.6.34.1+"patches in stable queue for next stable release". I tried
this some time ago with 2.6.33.x(don't remember which version exactly)
and it had the same problem, but at the time I stopped trying thinking
that it was a kvm problem. So basically there's no known(to me) good
version and no, I can't bisect this because this is my production
system. Anyway, I suspect this is reproducible. Am I the only one who
created a virtual hd file on a Btrfs and then used it with kvm/qemu? I
mean, it's not a particularly exotic test-case!


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