Btrfs: heuristic: add byte set calculation
Calculate byte set size for data sample:
- calculate how many unique bytes have been in the sample
- for all bytes count > 0, check if we're still in the low count range
(~25%), such data are easily compressible, otherwise furhter analysis
is needed
Signed-off-by: Timofey Titovets <[email protected]>
Reviewed-by: David Sterba <[email protected]>
[ update comments ]
Signed-off-by: David Sterba <[email protected]>
diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 0d445c8..e949f07 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -1222,6 +1222,45 @@ int btrfs_decompress_buf2page(const char *buf, unsigned long buf_start,
return 1;
}
+/*
+ * Count byte values in buckets.
+ * This heuristic can detect textual data (configs, xml, json, html, etc).
+ * Because in most text-like data byte set is restricted to limited number of
+ * possible characters, and that restriction in most cases makes data easy to
+ * compress.
+ *
+ * @BYTE_SET_THRESHOLD - consider all data within this byte set size:
+ * less - compressible
+ * more - need additional analysis
+ */
+#define BYTE_SET_THRESHOLD (64)
+
+static u32 byte_set_size(const struct heuristic_ws *ws)
+{
+ u32 i;
+ u32 byte_set_size = 0;
+
+ for (i = 0; i < BYTE_SET_THRESHOLD; i++) {
+ if (ws->bucket[i].count > 0)
+ byte_set_size++;
+ }
+
+ /*
+ * Continue collecting count of byte values in buckets. If the byte
+ * set size is bigger then the threshold, it's pointless to continue,
+ * the detection technique would fail for this type of data.
+ */
+ for (; i < BUCKET_SIZE; i++) {
+ if (ws->bucket[i].count > 0) {
+ byte_set_size++;
+ if (byte_set_size > BYTE_SET_THRESHOLD)
+ return byte_set_size;
+ }
+ }
+
+ return byte_set_size;
+}
+
static bool sample_repeated_patterns(struct heuristic_ws *ws)
{
const u32 half_of_sample = ws->sample_size / 2;
@@ -1321,6 +1360,12 @@ int btrfs_compress_heuristic(struct inode *inode, u64 start, u64 end)
ws->bucket[byte].count++;
}
+ i = byte_set_size(ws);
+ if (i < BYTE_SET_THRESHOLD) {
+ ret = 2;
+ goto out;
+ }
+
out:
__free_workspace(0, ws_list, true);
return ret;
