[dm-devel] [PATCH] dm-writecache: write at least 4k when committing

[Mikulas Patocka]

SSDs perform badly with sub-4k writes (because they perfrorm 
read-modify-write internally), so make sure that we write at least 4k when 
committing.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
Fixes: 991bd8d7bc78 ("dm writecache: commit just one block, not a full page")

Index: linux/drivers/md/dm-writecache.c
===================================================================
--- linux.orig/drivers/md/dm-writecache.c
+++ linux/drivers/md/dm-writecache.c
@@ -533,7 +533,11 @@ static void ssd_commit_superblock(struct
 
 	region.bdev = wc->ssd_dev->bdev;
 	region.sector = 0;
-	region.count = wc->block_size >> SECTOR_SHIFT;
+	region.count = max(4096U, wc->block_size) >> SECTOR_SHIFT;
+
+	if (unlikely(region.sector + region.count > wc->metadata_sectors))
+		region.count = wc->metadata_sectors - region.sector;
+
 	region.sector += wc->start_sector;
 
 	req.bi_op = REQ_OP_WRITE;