[Qemu-devel] -snapshot and tmpsfs

[Qemu-devel] -snapshot and tmpsfs

[Ottavio Caruso]

Are the writes to tmpfs (in a *nix guest OS) recorded to the
snapshotted image?

Example: /var/run on tmpfs
OS writes to /var/run. Bigger temp file or not?

Thank you

Ottavio


 
____________________________________________________________________________________
Sponsored Link

Compare mortgage rates for today. 
Get up to 5 free quotes. 
Www2.nextag.com

Re: [Qemu-devel] -snapshot and tmpsfs

[Michael McConnell]

On Sat, 18 Nov 2006, Ottavio Caruso wrote:

> Are the writes to tmpfs (in a *nix guest OS) recorded to the
> snapshotted image?
> 
> Example: /var/run on tmpfs
> OS writes to /var/run. Bigger temp file or not?

This isn't a guaranteed certain answer, but IIRC tmpfs uses system RAM and 
swap as its storage.  Therefore a guest writing into its tmpfs space would 
only touch the host filesystem if it were to use the swapfile.

As you say you're using the snapshot mode it would, if the swapfile is 
touched, go into the snapshot temp file, otherwise it wouldn't.

-- Michael "Soruk" McConnell
   Eridani Star System

   MailStripper - http://www.MailStripper.eu/ - SMTP spam filter
   Mail Me Anywhere - http://www.MailMeAnywhere.com/ - Mobile email

Re: [Qemu-devel] -snapshot and tmpsfs

[Mulyadi Santosa]

Hi Ottavio 
> Are the writes to tmpfs (in a *nix guest OS) recorded to the
> snapshotted image?
>
> Example: /var/run on tmpfs
> OS writes to /var/run. Bigger temp file or not?

>From what I saw, the temp file isn't bigger unless the tmpfs' size is 
exceeding the simulated RAM size, thus some of them are getting into 
the simulated swap...in this case, the snapshot image. 

Qemu itself already allocates a memory area as big as the argument you 
gave in -m. Here is something you see if you use -m 64 like I did:
42131000 (69968 KB)    rw-p (00:00 0)

(Because of the work of demand paging, this VMA size doesn't 
neccessarily reflect the size of the actual allocated pages).


I also try to diff the lsof's output and pmap's output before and after 
I write 43 MB of zero-filled file in tmpfs-based directory (while the 
size of tmpfs itself is 44MB) and I only get these:
# diff -u pmap{1,3}.txt
--- pmap1.txt   2006-11-19 16:31:44.000000000 +0700
+++ pmap3.txt   2006-11-19 16:33:54.000000000 +0700
@@ -1,7 +1,7 @@
 qemu(5017)
 08048000 (688 KB)      r-xp (16:02 897438)   
/usr/local/qemu080/bin/qemu
 080f4000 (36 KB)       rw-p (16:02 897438)   
/usr/local/qemu080/bin/qemu
-080fd000 (27372 KB)    rwxp (00:00 0)
+080fd000 (27384 KB)    rwxp (00:00 0)
 40000000 (84 KB)       r-xp (16:02 244393)   /lib/ld-2.3.2.so
 40015000 (4 KB)        rw-p (16:02 244393)   /lib/ld-2.3.2.so
 40016000 (4 KB)        rw-p (00:00 0)
@@ -36,4 +36,4 @@
 4212e000 (12 KB)       rw-p (16:02 651591)   /lib/tls/libc-2.3.2.so
 42131000 (69968 KB)    rw-p (00:00 0)
 bfff4000 (48 KB)       rwxp (00:00 0)
-mapped:   102288 KB writable/private: 98144 KB shared: 564 KB
+mapped:   102300 KB writable/private: 98156 KB shared: 564 KB

My conclusion: temp file will get bigger if we write something bigger 
than simulated RAM size (but still smaller than tmpfs size, of 
course..).

regards,

Mulyadi

Re: [Qemu-devel] -snapshot and tmpsfs

[Ottavio Caruso]

> From: Michael McConnell 
> On Sat, 18 Nov 2006, Ottavio Caruso wrote:
> 
> > Are the writes to tmpfs (in a *nix guest OS) recorded to the
> > snapshotted image?
> > 
> > Example: /var/run on tmpfs
> > OS writes to /var/run. Bigger temp file or not?
> 
> This isn't a guaranteed certain answer, but IIRC tmpfs uses system
> RAM and 
> swap as its storage.  Therefore a guest writing into its tmpfs
> space would 
> only touch the host filesystem if it were to use the swapfile.
> 
> As you say you're using the snapshot mode it would, if the swapfile
> is 
> touched, go into the snapshot temp file, otherwise it wouldn't.
> 
> -- Michael "Soruk" McConnell
>    Eridani Star System

As my guest OS (Debian) was filling /var/cache/apt/* with MB's of
stuff, I have modified /etc/fstab and mounted all /var/cache/apt and
children directories on tmpfs, then I have installed/uninstalled a
few packages. I have compared the size of the temp snapshot file with
the one generated before  and I have found no differences.
I wonder if that means something.

Ottavio Caruso


 
____________________________________________________________________________________
Sponsored Link

Mortgage rates near 39yr lows. 
$310k for $999/mo. Calculate new payment! 
www.LowerMyBills.com/lre