ircd::fs: Add write_options.alignment; padding out to alignment for aio write.

include/ircd/fs/write.h

@@ -44,6 +44,9 @@ struct ircd::fs::write_opts
 	/// Offset in the file to start the write from.
 	off_t offset {0};
 
+	/// Alignment requirement.
+	size_t alignment {0};
+
 	/// Request priority (this option may be improved, avoid for now)
 	int16_t priority {0};
 

ircd/aio.cc

@@ -13,6 +13,12 @@
 #include <ircd/asio.h>
 #include "aio.h"
 
+namespace ircd::fs
+{
+	extern char _zero_pads_[4096];
+	static size_t _zero_pads_required(const size_t &buf_size, const size_t &alignment);
+}
+
 //
 // request::fsync
 //
@@ -132,6 +138,10 @@ ircd::fs::aio::request::write::write(const int &fd,
 		const_cast<char *>(buffer::data(buf)),
 		buffer::size(buf)
 	},
+	{
+		_zero_pads_,
+		_zero_pads_required(buffer::size(buf), opts.alignment)
+	},
 }}
 {
 	aio_reqprio = opts.priority;
@@ -177,6 +187,36 @@ ircd::fs::prefetch__aio(const fd &fd,
 
 }
 
+//
+// internal util
+//
+
+decltype(ircd::fs::_zero_pads_)
+__attribute__((aligned(512)))
+ircd::fs::_zero_pads_
+{
+	0
+};
+
+size_t
+ircd::fs::_zero_pads_required(const size_t &buf_size,
+                              const size_t &alignment)
+{
+	if(!alignment || buf_size == alignment)
+		return 0;
+
+	if(unlikely(alignment > sizeof(_zero_pads_)))
+		throw error
+		{
+			make_error_code(std::errc::invalid_argument),
+			"Alignment %zu exceeds maximum of %zu",
+			alignment,
+			sizeof(_zero_pads_)
+		};
+
+	return alignment - (buf_size % alignment);
+}
+
 //
 // aio
 //

ircd/aio.h

@@ -94,7 +94,7 @@ struct ircd::fs::aio::request::read
 struct ircd::fs::aio::request::write
 :request
 {
-	std::array<struct iovec, 1> iov;
+	std::array<struct iovec, 2> iov;
 
 	write(const int &fd, const const_buffer &, const write_opts &);
 };