Re: [libvirt] [PATCH 7/8] save: add virDirectFd wrapper type

On 07/14/2011 08:24 AM, Eric Blake wrote: > O_DIRECT has stringent requirements - I/O must occur with buffers > that have both alignment and size as multiples of the file system > block size (used to be 512 bytes, but these days, 4k is safer, and > 64k allows for better throughput). Rather than make lots of changes > at each site that wants to use O_DIRECT, it is easier to offload > the work through a helper process that mirrors the I/O between a > pipe and the actual direct fd, so that the other end of the pipe > no longer has to worry about constraints. > > +virDirectFdPtr > +virDirectFdNew(int *fd, const char *name) > +{ > + virDirectFdPtr ret = NULL; > + bool output = false; > + int pipefd[2] = { -1, -1 }; > + int mode = -1; > + > + if (VIR_ALLOC(ret) < 0) { > + virReportOOMError(); > + goto error; > + } > + if (!O_DIRECT) > + return ret; Question - should an attempt to use 'virsh save --direct' on a system that lacks O_DIRECT (think mingw) be rejected, rather than silently ignored? My argument is that --direct is merely an optimization hint - it tries to reduce filesystem cache pollution (possibly at the expense of slower operation), but other than the cache effects, the end result is the same as if O_DIRECT is unavailable. Hence, my decision of silently ignoring it rather than erroring out.