while evaluatiing compressed qcow2 images as a good basis for
virtual machine templates I found out that there are a lot
of partly redundant (compressed clusters have common physical
sectors) and relatively short reads.
This doesn't hurt if the image resides on a local
filesystem where we can benefit from the local page cache,
but it adds a lot of penalty when accessing remote images
on NFS or similar exports.
This patch effectevily implements a readahead of 2 * cluster_size
which is 2 * 64kB per default resulting in 128kB readahead. This
is the common setting for Linux for instance.
For example this leads to the following times when converting
a compressed qcow2 image to a local tmpfs partition.
Old:
time ./qemu-img convert nfs://10.0.0.1/export/VC-Ubuntu-LTS-12.04.2-64bit.qcow2
/tmp/test.raw
real 0m24.681s
user 0m8.597s
sys 0m4.084s
New:
time ./qemu-img convert nfs://10.0.0.1/export/VC-Ubuntu-LTS-12.04.2-64bit.qcow2
/tmp/test.raw
real 0m16.121s
user 0m7.932s
sys 0m2.244s
Signed-off-by: Peter Lieven <p...@kamp.de>
---
block/qcow2-cluster.c | 27 +++++++++++++++++++++++++--
block/qcow2.h | 1 +
2 files changed, 26 insertions(+), 2 deletions(-)
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 11f9c50..367f089 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1321,7 +1321,7 @@ static int decompress_buffer(uint8_t *out_buf, int
out_buf_size,
int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset)
{
BDRVQcowState *s = bs->opaque;
- int ret, csize, nb_csectors, sector_offset;
+ int ret, csize, nb_csectors, sector_offset, max_read;
uint64_t coffset;
coffset = cluster_offset & s->cluster_offset_mask;
@@ -1329,9 +1329,32 @@ int qcow2_decompress_cluster(BlockDriverState *bs,
uint64_t cluster_offset)
nb_csectors = ((cluster_offset >> s->csize_shift) & s->csize_mask) + 1;
sector_offset = coffset & 511;
csize = nb_csectors * 512 - sector_offset;
+ max_read = MIN((bs->file->total_sectors - (coffset >> 9)), 2 *
s->cluster_sectors);
BLKDBG_EVENT(bs->file, BLKDBG_READ_COMPRESSED);
- ret = bdrv_read(bs->file, coffset >> 9, s->cluster_data, nb_csectors);
+ if (s->cluster_cache_offset != -1 && coffset > s->cluster_cache_offset
&&
+ (coffset >> 9) < (s->cluster_cache_offset >> 9) +
s->cluster_data_sectors) {
+ int cached_sectors = s->cluster_data_sectors - ((coffset >> 9) -
+ (s->cluster_cache_offset >> 9));
+ memmove(s->cluster_data,
+ s->cluster_data + (s->cluster_data_sectors -
cached_sectors) * 512,
+ cached_sectors * 512);
+ s->cluster_data_sectors = cached_sectors;
+ if (nb_csectors > cached_sectors) {
+ /* some sectors are missing read them and fill up to max_read
sectors */
+ ret = bdrv_read(bs->file, (coffset >> 9) + cached_sectors,
+ s->cluster_data + cached_sectors * 512,
+ max_read);
+ s->cluster_data_sectors = cached_sectors + max_read;
+ } else {
+ /* all relevant sectors are in the cache */
+ ret = 0;
+ }
+ } else {
+ ret = bdrv_read(bs->file, coffset >> 9, s->cluster_data, max_read);
+ s->cluster_data_sectors = max_read;
+ }
if (ret < 0) {
+ s->cluster_data_sectors = 0;
return ret;
}
if (decompress_buffer(s->cluster_cache, s->cluster_size,
diff --git a/block/qcow2.h b/block/qcow2.h
index 922e190..5edad26 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -185,6 +185,7 @@ typedef struct BDRVQcowState {
uint8_t *cluster_cache;
uint8_t *cluster_data;
+ int cluster_data_sectors;
uint64_t cluster_cache_offset;
QLIST_HEAD(QCowClusterAlloc, QCowL2Meta) cluster_allocs;
--
1.7.9.5
