On 2018-05-18 04:06, Anand Jain wrote:
Thanks Austin and Jeff for the suggestion.
I am not particularly a fan of mount option either mainly because
those options aren't persistent and host independent luns will
have tough time to have them synchronize manually.
Properties are better as it is persistent. And we can apply this
read_mirror_policy property on the fsid object.
But if we are talking about the properties then it can be stored
as extended attributes or ondisk key value pair, and I am doubt
if ondisk key value pair will get a nod.
I can explore the extended attribute approach but appreciate more
comments.
Hmm, thinking a bit further, might it be easier to just keep this as a
mount option, and add something that lets you embed default mount
options in the volume in a free-form manner? Then, you could set this
persistently there, and could specify any others you want too. Doing
that would also give very well defined behavior for exactly when changes
would apply (the next time you mount or remount the volume), though
handling of whether or not an option came from there or was specified on
the command-line might be a bit complicated.
On 05/17/2018 10:46 PM, Jeff Mahoney wrote:
On 5/17/18 8:25 AM, Austin S. Hemmelgarn wrote:
On 2018-05-16 22:32, Anand Jain wrote:
On 05/17/2018 06:35 AM, David Sterba wrote:
On Wed, May 16, 2018 at 06:03:56PM +0800, Anand Jain wrote:
Not yet ready for the integration. As I need to introduce
-o no_read_mirror_policy instead of -o read_mirror_policy=-<devid>
Mount option is mostly likely not the right interface for setting such
options, as usual.
I am ok to make it ioctl for the final. What do you think?
But to reproduce the bug posted in
Btrfs: fix the corruption by reading stale btree blocks
It needs to be a mount option, as randomly the pid can
still pick the disk specified in the mount option.
Personally, I'd vote for filesystem property (thus handled through the
standard `btrfs property` command) that can be overridden by a mount
option. With that approach, no new tool (or change to an existing tool)
would be needed, existing volumes could be converted to use it in a
backwards compatible manner (old kernels would just ignore the
property), and you could still have the behavior you want in tests (and
in theory it could easily be adapted to be a per-subvolume setting if we
ever get per-subvolume chunk profile support).
Properties are a combination of interfaces presented through a single
command. Although the kernel API would allow a direct-to-property
interface via the btrfs.* extended attributes, those are currently
limited to a single inode. The label property is set via ioctl and
stored in the superblock. The read-only subvolume property is also set
by ioctl but stored in the root flags.
As it stands, every property is explicitly defined in the tools, so any
addition would require tools changes. This is a bigger discussion,
though. We *could* use the xattr interface to access per-root or
fs-global properties, but we'd need to define that interface.
btrfs_listxattr could get interesting, though I suppose we could
simplify it by only allowing the per-subvolume and fs-global operations
on root inodes.
-Jeff
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