Re: [libvirt] [PATCH v2 02/12] qemu: let blockinfo reuse virStorageSource

Right now, grabbing blockinfo always calls stat on the disk, then opens the image to determine the capacity, using a throw-away virStorageSourcePtr. This has a couple of drawbacks: 1. We are calling stat and opening a file on every invocation of the API. However, there are cases where the stats should NOT be changing between successive calls (if a domain is running, no one should be changing the physical size of a block device or raw image behind our backs; capacity of read-only files should not be changing; and we are the gateway to the block-resize command to know when the capacity of read-write files should be changing). True, we still have to use stat in some cases (a sparse raw file changes allocation if it is read-write and the amount of holes is changing, and a read-write qcow2 image stored in a file changes physical size if it was not fully pre-allocated). But for read-only images, even this should be something we can remember from the previous time, rather than repeating every call. 2. We want to enhance the power of virDomainListGetStats, by sharing code. But we already have a virStorageSourcePtr for each disk, and it would be easier to reuse the common structure than to have to worry about the one-off virDomainBlockInfoPtr. While this patch does not optimize reuse of information in point 1, it does get us closer to being able to do so; by updating a structure that survives between consecutive calls. * src/util/virstoragefile.h (_virStorageSource): Add physical, to mirror virDomainBlockInfo; rearrange fields to match public struct. (virStorageSourceCopy): Copy the new field. * src/qemu/qemu_driver.c (qemuDomainGetBlockInfo): Store into storage source, then copy to block info. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; --- src/qemu/qemu_driver.c | 42 ++++++++++++++++++++++++++++++++++-------- src/util/virstoragefile.c | 3 ++- src/util/virstoragefile.h | 3 ++- 3 files changed, 38 insertions(+), 10 deletions(-) diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c index 8775ab2..b13c5e1 100644 --- a/src/qemu/qemu_driver.c +++ b/src/qemu/qemu_driver.c @@ -11045,6 +11045,26 @@ qemuDomainGetBlockInfo(virDomainPtr dom, disk = vm->def->disks[idx]; + /* FIXME: For an offline domain, we always want to check current + * on-disk statistics (as users have been known to change offline + * images behind our backs). For a running domain, however, it + * would be nice to avoid opening a file (particularly since + * reading a file while qemu is writing it risks the reader seeing + * bogus data), or even avoid a stat, if the information + * remembered from the previous run is still viable. + * + * For read-only disks, nothing should be changing unless the user + * has requested a block-commit action. For read-write disks, we + * know some special cases: capacity should not change without a + * block-resize (where capacity is the only stat that requires + * opening a file, and even then, only for non-raw files); and + * physical size of a raw image or of a block device should + * likewise not be changing without block-resize. On the other + * hand, allocation of a raw file can change (if the file is + * sparse, but the amount of sparseness changes due to writes or + * punching holes), and physical size of a non-raw file can + * change. + */

if (virStorageSourceIsLocalStorage(disk->src)) { if (!disk->src->path) { virReportError(VIR_ERR_INVALID_ARG,