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.
--
Eric Blake eblake(a)redhat.com +1-801-349-2682
Libvirt virtualization library http://libvirt.org
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