Port-xen archive

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Old Index]

Re: Instability with NetBSD Dom0

tg%gmplib.org@localhost (Torbjörn Granlund) writes:

> This actually sounds like a plausible explanation.  Thank you!
> I've seen Dom0 CPU usage go up inexplicably for certain DomU file
> operations.  Usually the systems stabilise after having been frozen for
> perhaps a dozen seconds.  I recall that I some years ago stopped using
> sparse files under NetBSD, but I cannot recall the details of what led
> me to do that.
> The new disastrous problem happened when I used a 20 GB sparse file
> (yes, qemu-img-xen creates sparse files); all images used for this
> system were just 1 GB to 2 GB.  Perhaps size matters?

I think the basic issue is that a write arriving at the vnd which then
needs to do fs ops to allocate a block can hang.  But I'm really unclear
on the details.

> I'll try and find time for some experiments.  (I will need to remove the
> system from the co-location site.)

I'm not sure why you want to experiment :-) Seriously, on a production
machine, I would recommend just making all files used for vnd
non-sparse.  (If you want to grab another box to experiment on a home to
fix the bug, great, but I would decouple that from your production

Another reason not to use sparse files is that creating files non-sparse
gets you pretty good locality of block allocation and I think this leads
to higher efficiency, but I don't have test data.

So I'd just copy each image file with dd, and then rm the old one and
rename.  As in just:

  dd if=image of=image.new bs=1m
  mv image image.sparse
  mv image.new image

and then delete image.sparse when you are confident.

(note to self: fix the howto to explain this!)


Attachment: pgpn90J4slNTi.pgp
Description: PGP signature

Home | Main Index | Thread Index | Old Index