23278c81fb
It's to ensure the data integrity during unexpected power failure.
Background:
Since release 1.3, in Linux system, containerD unpacks and writes files into
overlayfs snapshot directly. It doesn’t involve any mount-umount operations
so that the performance of pulling image has been improved.
As we know, the umount syscall for overlayfs will force kernel to flush
all the dirty pages into disk. Without umount syscall, the files’ data relies
on kernel’s writeback threads or filesystem's commit setting (for
instance, ext4 filesystem).
The files in committed snapshot can be loss after unexpected power failure.
However, the snapshot has been committed and the metadata also has been
fsynced. There is data inconsistency between snapshot metadata and files
in that snapshot.
We, containerd, received several issues about data loss after unexpected
power failure.
* https://github.com/containerd/containerd/issues/5854
* https://github.com/containerd/containerd/issues/3369#issuecomment-1787334907
Solution:
* Option 1: SyncFs after unpack
Linux platform provides [syncfs][syncfs] syscall to synchronize just the
filesystem containing a given file.
* Option 2: Fsync directories recursively and fsync on regular file
The fsync doesn't support symlink/block device/char device files. We
need to use fsync the parent directory to ensure that entry is
persisted.
However, based on [xfstest-dev][xfstest-dev], there is no case to ensure
fsync-on-parent can persist the special file's metadata, for example,
uid/gid, access mode.
Checkout [generic/690][generic/690]: Syncing parent dir can persist
symlink. But for f2fs, it needs special mount option. And it doesn't say
that uid/gid can be persisted. All the details are behind the
implemetation.
> NOTE: All the related test cases has `_flakey_drop_and_remount` in
[xfstest-dev].
Based on discussion about [Documenting the crash-recovery guarantees of Linux file systems][kernel-crash-recovery-data-integrity],
we can't rely on Fsync-on-parent.
* Option 1 is winner
This patch is using option 1.
There is test result based on [test-tool][test-tool].
All the networking traffic created by pull is local.
* Image: docker.io/library/golang:1.19.4 (992 MiB)
* Current: 5.446738579s
* WIOS=21081, WBytes=1329741824, RIOS=79, RBytes=1197056
* Option 1: 6.239686088s
* WIOS=34804, WBytes=1454845952, RIOS=79, RBytes=1197056
* Option 2: 1m30.510934813s
* WIOS=42143, WBytes=1471397888, RIOS=82, RBytes=1209344
* Image: docker.io/tensorflow/tensorflow:latest (1.78 GiB, ~32590 Inodes)
* Current: 8.852718042s
* WIOS=39417, WBytes=2412818432, RIOS=2673, RBytes=335987712
* Option 1: 9.683387174s
* WIOS=42767, WBytes=2431750144, RIOS=89, RBytes=1238016
* Option 2: 1m54.302103719s
* WIOS=54403, WBytes=2460528640, RIOS=1709, RBytes=208237568
The Option 1 will increase `wios`. So, the `image_pull_with_sync_fs` is
option in CRI plugin.
[syncfs]: <https://man7.org/linux/man-pages/man2/syncfs.2.html>
[xfstest-dev]: <https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git>
[generic/690]: <https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/tree/tests/generic/690?h=v2023.11.19>
[kernel-crash-recovery-data-integrity]: <https://lore.kernel.org/linux-fsdevel/1552418820-18102-1-git-send-email-jaya@cs.utexas.edu/>
[test-tool]: <
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| .. | ||
| ansible | ||
| apparmor | ||
| autocomplete | ||
| aws | ||
| diffservice | ||
| Dockerfile.test.d | ||
| fuzz | ||
| gce | ||
| nvidia | ||
| seccomp | ||
| snapshotservice | ||
| Dockerfile.test | ||
| README.md | ||
contrib
The contrib directory contains packages that do not belong in the core containerd packages but still contribute to overall containerd usability.
Package such as Apparmor or Selinux are placed in contrib because they are platform dependent and often require higher level tools and profiles to work.
Packaging and other built tools can be added to contrib to aid in packaging containerd for various distributions.
Testing
Code in the contrib directory may or may not have been tested in the normal test pipeline for core components.