Re: NetBSD+Xen with short-lived throw-away guests

[Stephan <stephanwib%googlemail.com@localhost>]

What we need is something like Citrix Provisioning Services.


Regards,

Stephan

2014-03-05 13:16 GMT+01:00 Torbjorn Granlund <tg%gmplib.org@localhost>:
> I am developing a compiler teaching/judging system based on NetBSD+Xen
> (as Dom0) where all execution of student compilers is done in Xen guests
> (DomU).  For executing the code the student compilers generate from our
> test suite, we use either Xen (for x86 and JVM targets) or qemu (for
> ARM, SPARC, MIPS targets).
>
> Before booting a Xen guest, a prepared master disk image is copied to a
> throw-away image.  The throw-away image is used for one judging round,
> then discarded.
>
> The Xen master disk images are about 4 GB each.  They could be trimmed,
> but not to a really small size.
>
> My first attempt was to create a tmpfs for the throw-away image, then cp
> from a master image to the tmpfs.  This copying is fast, more so since
> the master image tend to live in buffer cache.
>
> But to my surprise the NetBSD guest then runs poorly.  It creates 100%
> load the Dom0 for several minutes while booting.  This DomU uses PV
> virtualisation (in fact it uses the kernel made available for this
> purpose by the NetBSD project, version 6.1.3).
>
> Attempt 2 is more plain.  I just cp (or dd) the master image to a
> throw-away image in the same filesystem.  The disk is an Intel S3500 SSD
> (or actually two mirrored S3500).  This copy procedure takes about two
> minutes, and also wears on the SSD (4 GB is written every 5 minutes
> under full load).  But this naive solution is faster than the tmpfs
> trick.
>
> I haven't tried replacing tmpfs with mfs.  I haven't tried using HVM
> instead of PV.
>
> Do you have any advice on how to do this cleverly?
>
> A copy-on-write on a per-file basis would be awesome...
>
> The system is operational and used already, I just want to improve its
> speed and decrease SSD wear.
>
>
> Torbjörn
>