Scramble on a 512b boundary
Instead of just scrambling the full block at the beginning at the
end, make it a bit more clever by:
- Doing it at 512b intervals to ensure full 512b de-dupe attemts, and
- Do it at a "random" offset into that block.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/io_u.c b/io_u.c
index af2d05f..f4c4aa2 100644
--- a/io_u.c
+++ b/io_u.c
@@ -1122,15 +1122,35 @@
/*
* Fill offset and start time into the buffer content, to prevent too
- * easy compressible data for simple de-dupe attempts.
+ * easy compressible data for simple de-dupe attempts. Do this for every
+ * 512b block in the range, since that should be the smallest block size
+ * we can expect from a device.
*/
static void small_content_scramble(struct io_u *io_u)
{
- void *end;
+ unsigned int i, nr_blocks = io_u->buflen / 512;
+ unsigned int offset;
+ void *p, *end;
- *((unsigned long long *) io_u->xfer_buf) = io_u->offset;
- end = io_u->xfer_buf + io_u->xfer_buflen - sizeof(io_u->start_time);
- memcpy(end, &io_u->start_time, sizeof(io_u->start_time));
+ if (!nr_blocks)
+ return;
+
+ p = io_u->xfer_buf;
+ for (i = 0; i < nr_blocks; i++) {
+ /*
+ * Fill the byte offset into a "random" start offset of
+ * the buffer, given by the product of the usec time
+ * and the actual offset.
+ */
+ offset = (io_u->start_time.tv_usec * io_u->offset) & 511;
+ if (offset >= 512 - sizeof(unsigned long long))
+ offset -= sizeof(unsigned long long);
+ *((unsigned long long *) p + offset) = io_u->offset;
+
+ end = p + 512 - sizeof(io_u->start_time);
+ memcpy(end, &io_u->start_time, sizeof(io_u->start_time));
+ p += 512;
+ }
}
/*