On 2018-08-29 08:33, Nikolay Borisov wrote:
On 29.08.2018 15:09, Qu Wenruo wrote:
On 2018/8/29 下午4:35, Nikolay Borisov wrote:
Here is the userspace tooling support for utilising the new metadata_uuid field,
enabling the change of fsid without having to rewrite every metadata block. This
patchset consists of adding support for the new field to various tools and
files (Patch 1). The actual implementation of the new -m|-M options (which are
described in more detail in Patch 2). A new misc-tests testcasei (Patch 3) which
exercises the new options and verifies certain invariants hold (these are also
described in Patch2). Patch 4 is more or less copy of the kernel conuterpart
just reducing some duplication between btrfs_fs_info and btrfs_fs_devices
structures.
So to my understand, now we have another layer of UUID.
Before we have one fsid, both used in superblock and tree blocks.
Now we have 2 fsid, the one used in tree blocks are kept the same, but
changed its name to metadata_uuid in superblock.
And superblock::fsid will become a new field, and although they are the
same at mkfs time, they could change several times during its operation.
This indeed makes uuid change super fast, only needs to update all
superblocks of the fs, instead of all tree blocks.
However I have one nitpick of the design. Unlike XFS, btrfs supports
multiple devices.
If we have a raid10 fs with 4 devices, and it has already gone through
several UUID change (so its metadata uuid is already different from fsid).
And during another UUID change procedure, we lost power while only
updated 2 super blocks, what will happen for kernel device assembly?
(Although considering how fast the UUID change would happen, such case
should be super niche)
Then I guess you will be fucked. I'm all ears for suggestion how to
rectify this without skyrocketing the complexity. The current UUID
rewrite method sets a flag int he superblock that FSID change is in
progress and clears it once every metadatablock has been rewritten. I
can piggyback on this mechanism but I'm not sure it provides 100%
guarantee. Because by the some token you can set this flag, start
writing the super blocks then lose power and then only some of the
superblocks could have this flag set so we back at square 1.
The intended usecase of this feature is to give the sysadmin the ability to
create copies of filesystesm, change their uuid quickly and mount them alongside
the original filesystem for, say, forensic purposes.
One thing which still hasn't been set in stone is whether the new options
will remain as -m|-M or whether they should subsume the current -u|-U - from
the point of view of users nothing should change.
Well, user would be surprised by how fast the new -m is, thus there is
still something changed :)
I prefer to subsume current -u/-U, and use the new one if the incompat
feature is already set. Or fall back to original behavior.
But I'm not a fan of using INCOMPAT flags as an indicator of changed
fsid/metadata uuid.
INCOMPAT feature should not change so easily nor acts as an indicator.
That's to say, the flag should only be set at mkfs time, and then never
change unlike the 2nd patch (I don't even like btrfstune to change
incompat flags).
E.g.
mkfs.btrfs -O metadata_uuid <device>, then we could use the new way to
change fsid without touching metadata uuid.
Or we could only use the old method.
I disagree, I don't see any benefit in this but only added complexity.
Can you elaborate more ?
Same here, I see essentially zero benefit to this, and one _big_
drawback, namely that you can't convert an existing volume to use this
approach if it's a feature that can only be set at mkfs time.
That one drawback means that this is effectively useless for all
existing BTRFS volumes, which is a pretty big limitation.
I also do think an INCOMPAT feature bit is appropriate here. Volumes
with this feature will potentially be enumerated with the wrong UUID on
older kernels, which is a pretty big behavioral issue (on the level of
completely breaking boot on some systems, keep in mind that almost all
major distros use volume UUID's to identify volumes in /etc/fstab).
Thanks,
Qu
So this is something which
I'd like to hear from the community. Of course the alternative of rewriting
the metadata blocks will be assigne new options - perhaps -m|M ?
I've tested this with multiple xfstest runs with the new tools installed as
well as running btrfs-progs test and have observed no regressions.
Nikolay Borisov (4):
btrfs-progs: Add support for metadata_uuid field.
btrfstune: Add support for changing the user uuid
btrfs-progs: tests: Add tests for changing fsid feature
btrfs-progs: Remove fsid/metdata_uuid fields from fs_info
btrfstune.c | 174 ++++++++++++++++++++++++-----
check/main.c | 2 +-
chunk-recover.c | 17 ++-
cmds-filesystem.c | 2 +
cmds-inspect-dump-super.c | 22 +++-
convert/common.c | 2 +
ctree.c | 15 +--
ctree.h | 8 +-
disk-io.c | 62 ++++++++--
image/main.c | 25 +++--
tests/misc-tests/033-metadata-uuid/test.sh | 137 +++++++++++++++++++++++
volumes.c | 37 ++++--
volumes.h | 1 +
13 files changed, 431 insertions(+), 73 deletions(-)
create mode 100755 tests/misc-tests/033-metadata-uuid/test.sh
