$ strace -cf x86_64-softmmu/qemu-system-x86_64 test.raw
Uncached getlength:
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
96.40 1.944174 13136 148 4 futex
1.65 0.033259 1 56418 2507 select
0.39 0.007817 0 81118 5561 read
0.33 0.006556 0 78787 timer_gettime
0.31 0.006223 0 56412 timer_settime
0.26 0.005191 0 47723 lseek
0.24 0.004924 0 51896 write
0.24 0.004800 0 51844 2917 rt_sigreturn
0.17 0.003333 833 4 shmdt
0.01 0.000175 0 790 poll
Cached getlength:
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
97.25 2.266124 14715 154 4 futex
1.03 0.023984 0 57749 3200 select
0.37 0.008644 0 79926 timer_gettime
0.29 0.006761 0 82390 6601 read
0.27 0.006398 0 57900 timer_settime
0.26 0.006038 0 52503 write
0.26 0.005985 0 52450 3671 rt_sigreturn
0.15 0.003418 1139 3 shmdt
0.10 0.002398 0 23846 lseek
0.01 0.000216 3 81 4 open
I think there are still a lot of lseeks left because
raw-posix.c:raw_pread_aligned() is implemented using lseek+read
instead of pread. Does anyone know the reasoning there or could
pread() be used?
Here is the cached getlength hack (I'm not confident that this patch
is correct in all cases, just a quick experiment):
diff --git a/block.c b/block.c
index 0f6be17..447327f 100644
--- a/block.c
+++ b/block.c
@@ -957,13 +957,25 @@ int bdrv_pwrite(BlockDriverState *bs, int64_t offset,
int bdrv_truncate(BlockDriverState *bs, int64_t offset)
{
BlockDriver *drv = bs->drv;
+ int ret;
if (!drv)
return -ENOMEDIUM;
if (!drv->bdrv_truncate)
return -ENOTSUP;
if (bs->read_only)
return -EACCES;
- return drv->bdrv_truncate(bs, offset);
+ ret = drv->bdrv_truncate(bs, offset);
+ if (ret < 0) {
+ return ret;
+ }
+
+ /* refresh total sectors */
+ if (drv->bdrv_getlength) {
+ bs->total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
+ } else {
+ bs->total_sectors = offset >> BDRV_SECTOR_BITS;
+ }
+ return ret;
}
/**
@@ -974,8 +986,12 @@ int64_t bdrv_getlength(BlockDriverState *bs)
BlockDriver *drv = bs->drv;
if (!drv)
return -ENOMEDIUM;
- if (!drv->bdrv_getlength) {
- /* legacy mode */
+
+ /* Fixed size devices use the total_sectors value for speed instead of
+ issuing a length query (like lseek) on each call. Also, legacy block
+ drivers don't provide a bdrv_getlength function and must use
+ total_sectors. */
+ if ((bs->total_sectors && !bs->growable) || !drv->bdrv_getlength) {
return bs->total_sectors * BDRV_SECTOR_SIZE;
}
return drv->bdrv_getlength(bs);
Stefan
