I have a architecture-esque question: When a file-backed(loopback, not blktap) PV domU writes, I have heard that it does not write directly to disk and is buffered somewhere. Where then, is it buffered? Is it in the dom0 loopback driver(mem), the dom0 filesystem cache(mem) or somewhere else? Thanks Chris _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
> -----Original Message----- > From: xen-users-bounces@lists.xensource.com > [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of > Chris Moore > Sent: 08 June 2007 20:18 > To: xen-users@lists.xensource.com > Subject: [Xen-users] loopback domU write > > > I have a architecture-esque question: > > When a file-backed(loopback, not blktap) PV domU writes, I > have heard that it does not write directly to disk and is > buffered somewhere. > Where then, is it buffered? Is it in the dom0 loopback > driver(mem), the dom0 filesystem cache(mem) or somewhere else?The dom0 filesystem cache will DEFINITELY be a part of the caching/buffering. I don''t know if the loopback driver also has some caches - it seems silly to cache here (because it is better to have one (larger) cache in one place (compared to multiple smaller caches), and loopback ends up in a filesystem driver sooner or later), but it''s obviously POSSIBLE to do. -- Mats> > Thanks > Chris > > >_______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
On Tue, Jun 12, 2007 at 06:57:11PM +0200, Petersson, Mats wrote:> > > > -----Original Message----- > > From: xen-users-bounces@lists.xensource.com > > [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of > > Chris Moore > > Sent: 08 June 2007 20:18 > > To: xen-users@lists.xensource.com > > Subject: [Xen-users] loopback domU write > > > > > > I have a architecture-esque question: > > > > When a file-backed(loopback, not blktap) PV domU writes, I > > have heard that it does not write directly to disk and is > > buffered somewhere. > > Where then, is it buffered? Is it in the dom0 loopback > > driver(mem), the dom0 filesystem cache(mem) or somewhere else? > > The dom0 filesystem cache will DEFINITELY be a part of the > caching/buffering. I don''t know if the loopback driver also has some > caches - it seems silly to cache here (because it is better to have one > (larger) cache in one place (compared to multiple smaller caches), and > loopback ends up in a filesystem driver sooner or later), but it''s > obviously POSSIBLE to do.loopback does indeed cache data for an arbitrarily long time. This is the primary reason why you should *never* use loopback under any circumstance. It does not honour guest requests to flush data to disk, so if your Dom0 crashes say goodbye vast amounts of data from your DomU filesystems. Always use blktap for file backed guests if you care about your data in the slightest. BTW is this only a paravirt problem really - HVM guests don''t use the loopback driver as QEMU handles writing to files/devices natively without needing loopback devices. Dan. -- |=- Red Hat, Engineering, Emerging Technologies, Boston. +1 978 392 2496 -=| |=- Perl modules: http://search.cpan.org/~danberr/ -=| |=- Projects: http://freshmeat.net/~danielpb/ -=| |=- GnuPG: 7D3B9505 F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 -=| _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
Daniel P. Berrange wrote:> DomU filesystems. Always use blktap for file backed guests if you > care about your data in the slightest. BTW is this only a paravirtInteresting.. I though blktap == lvm..? How do you invoke it for file backed devices? Tony _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
Tony Hoyle wrote:> Daniel P. Berrange wrote: >> DomU filesystems. Always use blktap for file backed guests if you >> care about your data in the slightest. BTW is this only a paravirt > > Interesting.. I though blktap == lvm..? How do you invoke it for file > backed devices?Ahh scratch that blktap is specifically disabled in the debian kernels - looks like the devs have issues with it as there''s a patch to remove it from the xen kernels they ship - http://svn.debian.org/wsvn/pkg-xen/trunk/xen-3.0/debian/patches/blktap-disable.dpatch I guess they have their reasons.. I''ll stick to the standard loopback devices. Tony _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users