On Tue, Mar 1, 2011 at 6:02 PM, Aneesh Kumar K. V <aneesh.ku...@linux.vnet.ibm.com> wrote: > On Tue, 1 Mar 2011 15:59:19 +0000, Stefan Hajnoczi <stefa...@gmail.com> wrote: >> >> Please explain the semantics of P9_TSYNCFS. Won't returning success >> >> without doing anything lead to data integrity issues? >> > >> > I should actually do the 9P Operation format as commit message. Will >> > add in the next update. Whether returning here would cause a data >> > integrity issue, it depends what sort of guarantee we want to >> > provide. So calling sync on the guest will cause all the dirty pages in >> > the guest to be flushed to host. Now all those changes are in the host >> > page cache and it would be nice to flush them as a part of sync but >> > then since we don't have a per file system sync, the above would imply >> > we flush all dirty pages on the host which can result in large >> > performance impact. >> >> You get the define the semantics of P9_TSYNCFS? I thought this is >> part of a well-defined protocol :). If this is a .L extension then >> it's probably a bad design and shouldn't be added to the protocol if >> we can't implement it. > > It is a part of .L extension and we can definitely implement it. There > is patch out there which is yet to be merged > > http://thread.gmane.org/gmane.linux.file-systems/44628
A future Linux-only ioctl :/. >> Is this operation supposed to flush the disk write cache too? > > I am not sure we need to specify that as a part of 9p operation. I guess > we can only say maximum possible data integrity. Whether a sync will > cause disk write cache flush depends on the file system. For ext* that > can be controlled by mount option barrier. So on a host with a safe configuration this operation should put data on stable storage? >> >> I think virtio-9p has a file descriptor cache. Would it be possible >> to fsync() those file descriptors? >> > > Ideally we should. But that would involve a large number of fsync calls. Yep, that's why this is a weird operation to support, especially since it's a .L add-on and not original 9P. What's the use-case since today's Linux userland cannot directly make use of this operation? I guess it has been added in order to pass-through a Linux internal vfs super block sync function? Stefan
