Re: [Qemu-devel] [PATCH v5 0/2] block: enforce minimal 4096 alignment in qemu_blockalign

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/05/2015 18:46, Denis V. Lunev wrote:
> I agree with this. Kernel guys are aware and may be we will have
> the fix after a while... I have heard (not tested) that performance
> loss over multi-queue SSD is around 30%.

I came up with this patch... can you test it with your test case
(and old QEMU) to see if it works for you?  I don't see much
improvement, but neither do I see it with new QEMU.

Paolo

diff --git a/block/bio.c b/block/bio.c
index f66a4eae16ee..df5bde7ebded 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -705,6 +705,7 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 {
 	int retried_segments = 0;
 	struct bio_vec *bvec;
+	int speculative_len;
 
 	/*
 	 * cloned bio must not modify vec list
@@ -712,7 +713,16 @@ static int __bio_add_page(struct request_queue *q, struct bio *bio, struct page
 	if (unlikely(bio_flagged(bio, BIO_CLONED)))
 		return 0;
 
-	if (((bio->bi_iter.bi_size + len) >> 9) > max_sectors)
+	/*
+	 * If the bio is not page-aligned, stop if we cannot fit this entire
+	 * page in the vector.  This is a very large write, so we'd like
+	 * to split it so as to keep the remaining bios aligned.
+	 */
+	speculative_len = len;
+	if (bio->bi_iter.bi_size & (PAGE_SIZE - 1) && offset == 0)
+		speculative_len = PAGE_SIZE;
+
+	if (((bio->bi_iter.bi_size + speculative_len) >> 9) > max_sectors)
 		return 0;
 
 	/*