On Fri, Oct 03, 2014 at 02:12:06PM +1000, Alexey Kardashevskiy wrote: > BDRV_O_INCOMING is only set when QEMU is about to receive migration and > we do not want QEMU to check the file at opening time as there is likely > garbage. Is there any other use of BDRV_O_INCOMING? There must be some as > bdrv_clear_incoming_migration_all() is called at the end of live > migration and I believe there must be a reason for it (cache does not > migrate, does it?).
BDRV_O_INCOMING is just for live migration. The cached data is not migrated, this is why it must be refreshed upon migration handover. > bdrv_invalidate_cache() flushes cache as it could be already initialized > even if QEMU is receiving migration - QEMU could have cached some of real > disk data. Is that correct? I do not really understand why it would > happen if there is BDRV_O_INCOMING set but ok. .bdrv_open() can load metadata from image files (such as the qcow2 L1 tables) and it does this even when BDRV_O_INCOMING is set. That data needs to be re-read at migration handover to prevent the destination QEMU from running with stale image metadata. > diff --git a/nbd.c b/nbd.c > index e9b539b..953c378 100644 > - --- a/nbd.c > +++ b/nbd.c > @@ -972,6 +972,7 @@ NBDExport *nbd_export_new(BlockDriverState *bs, off_t > dev_offset, > exp->ctx = bdrv_get_aio_context(bs); > bdrv_ref(bs); > bdrv_add_aio_context_notifier(bs, bs_aio_attached, bs_aio_detach, exp); > + bdrv_invalidate_cache(bs, NULL); > return exp; > } Please add a comment to explain why this call is necessary: /* NBD exports are used for non-shared storage migration. Make sure * that BDRV_O_INCOMING is cleared and the image is ready for write * access since the export could be available before migration handover. */ If someone can come up with a 2- or 3-line comment that explains it better, great. The rest of the patch looks like it will work. I'm not thrilled about putting BDRV_O_INCOMING logical inside bdrv_invalidate_cache() because there was no coupling there before, but the code seems correct now.
pgpmK5p8VuhKd.pgp
Description: PGP signature
