| Message ID | 20201204093103.9878-1-andrey.gruzdev@virtuozzo.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | migration: UFFD write-tracking migration/snapshots | expand |
On Fri, Dec 04, 2020 at 12:30:59PM +0300, Andrey Gruzdev wrote: > This patch series is a kind of 'rethinking' of Denis Plotnikov's ideas he's > implemented in his series '[PATCH v0 0/4] migration: add background snapshot'. > > Currently the only way to make (external) live VM snapshot is using existing > dirty page logging migration mechanism. The main problem is that it tends to > produce a lot of page duplicates while running VM goes on updating already > saved pages. That leads to the fact that vmstate image size is commonly several > times bigger then non-zero part of virtual machine's RSS. Time required to > converge RAM migration and the size of snapshot image severely depend on the > guest memory write rate, sometimes resulting in unacceptably long snapshot > creation time and huge image size. > > This series propose a way to solve the aforementioned problems. This is done > by using different RAM migration mechanism based on UFFD write protection > management introduced in v5.7 kernel. The migration strategy is to 'freeze' > guest RAM content using write-protection and iteratively release protection > for memory ranges that have already been saved to the migration stream. > At the same time we read in pending UFFD write fault events and save those > pages out-of-order with higher priority. > > How to use: > 1. Enable write-tracking migration capability > virsh qemu-monitor-command <domain> --hmp migrate_set_capability. > track-writes-ram on > > 2. Start the external migration to a file > virsh qemu-monitor-command <domain> --hmp migrate exec:'cat > ./vm_state' > > 3. Wait for the migration finish and check that the migration has completed. > state. > > Changes v4->v5: > > * 1. Refactored util/userfaultfd.c code to support features required by postcopy. > * 2. Introduced checks for host kernel and guest memory backend compatibility > * to 'background-snapshot' branch in migrate_caps_check(). > * 3. Switched to using trace_xxx instead of info_report()/error_report() for > * cases when error message must be hidden (probing UFFD-IO) or info may be > * really littering output if goes to stderr. > * 4 Added RCU_READ_LOCK_GUARDs to the code dealing with RAM block list. > * 5. Added memory_region_ref() for each RAM block being wr-protected. > * 6. Reused qemu_ram_block_from_host() instead of custom RAM block lookup routine. > * 7. Refused from using specific hwaddr/ram_addr_t in favour of void */uint64_t. > * 8. Currently dropped 'linear-scan-rate-limiting' patch. The reason is that > * that choosen criteria for high-latency fault detection (i.e. timestamp of > * UFFD event fetch) is not representative enough for this task. > * At the moment it looks somehow like premature optimization effort. > * 8. Dropped some unnecessary/unused code. I went over the series and it looks nice! There're a few todos for this series, so I added them into the wiki page (I created a "feature" section for migration todo and put live snapshot there): https://wiki.qemu.org/ToDo/LiveMigration#Features Anyone feel free to add.. Thanks,
On 08.12.2020 21:24, Peter Xu wrote: > On Fri, Dec 04, 2020 at 12:30:59PM +0300, Andrey Gruzdev wrote: >> This patch series is a kind of 'rethinking' of Denis Plotnikov's ideas he's >> implemented in his series '[PATCH v0 0/4] migration: add background snapshot'. >> >> Currently the only way to make (external) live VM snapshot is using existing >> dirty page logging migration mechanism. The main problem is that it tends to >> produce a lot of page duplicates while running VM goes on updating already >> saved pages. That leads to the fact that vmstate image size is commonly several >> times bigger then non-zero part of virtual machine's RSS. Time required to >> converge RAM migration and the size of snapshot image severely depend on the >> guest memory write rate, sometimes resulting in unacceptably long snapshot >> creation time and huge image size. >> >> This series propose a way to solve the aforementioned problems. This is done >> by using different RAM migration mechanism based on UFFD write protection >> management introduced in v5.7 kernel. The migration strategy is to 'freeze' >> guest RAM content using write-protection and iteratively release protection >> for memory ranges that have already been saved to the migration stream. >> At the same time we read in pending UFFD write fault events and save those >> pages out-of-order with higher priority. >> >> How to use: >> 1. Enable write-tracking migration capability >> virsh qemu-monitor-command <domain> --hmp migrate_set_capability. >> track-writes-ram on >> >> 2. Start the external migration to a file >> virsh qemu-monitor-command <domain> --hmp migrate exec:'cat > ./vm_state' >> >> 3. Wait for the migration finish and check that the migration has completed. >> state. >> >> Changes v4->v5: >> >> * 1. Refactored util/userfaultfd.c code to support features required by postcopy. >> * 2. Introduced checks for host kernel and guest memory backend compatibility >> * to 'background-snapshot' branch in migrate_caps_check(). >> * 3. Switched to using trace_xxx instead of info_report()/error_report() for >> * cases when error message must be hidden (probing UFFD-IO) or info may be >> * really littering output if goes to stderr. >> * 4 Added RCU_READ_LOCK_GUARDs to the code dealing with RAM block list. >> * 5. Added memory_region_ref() for each RAM block being wr-protected. >> * 6. Reused qemu_ram_block_from_host() instead of custom RAM block lookup routine. >> * 7. Refused from using specific hwaddr/ram_addr_t in favour of void */uint64_t. >> * 8. Currently dropped 'linear-scan-rate-limiting' patch. The reason is that >> * that choosen criteria for high-latency fault detection (i.e. timestamp of >> * UFFD event fetch) is not representative enough for this task. >> * At the moment it looks somehow like premature optimization effort. >> * 8. Dropped some unnecessary/unused code. > I went over the series and it looks nice! > > There're a few todos for this series, so I added them into the wiki page (I > created a "feature" section for migration todo and put live snapshot there): > > https://wiki.qemu.org/ToDo/LiveMigration#Features > > Anyone feel free to add.. > > Thanks, > Thanks, Peter!