+ * Writes are handled in three different ways:
+ *
+ * WR_INDIRECT:
+ * In this mode, if we need to commit the write later, then the block
+ * is immediately written into the file system (using dmu_sync),
+ * and a pointer to the block is put into the log record.
+ * When the txg commits the block is linked in.
+ * This saves additionally writing the data into the log record.
+ * There are a few requirements for this to occur:
+ * - write is greater than zfs/zvol_immediate_write_sz
+ * - not using slogs (as slogs are assumed to always be faster
+ * than writing into the main pool)
+ * - the write occupies only one block
+ * WR_COPIED:
+ * If we know we'll immediately be committing the
+ * transaction (FSYNC or FDSYNC), the we allocate a larger
+ * log record here for the data and copy the data in.
+ * WR_NEED_COPY:
+ * Otherwise we don't allocate a buffer, and *if* we need to
+ * flush the write later then a buffer is allocated and
+ * we retrieve the data using the dmu.