[Gluster-devel] DHT: Making Dir rename op's crash consistent

[Shishir Gowda]

Hi All,

This is a proposed enhancement to DHT directory rename operations to make it 
recoverable in-case of crashes.

Please feel free to review/comment on the design. There also 2 open issues 
which need to tackled (see below in recovery logic)

We propose to add 2 new on-disk xattrs SRC(<key to be decided>:destination 
path) and DST(<key to be decided>:src path/gfid).

Consider these scenarios

case1. Only source directory exists
case2. Both source, and destination directories exist.

The tasks for rename would be as follows:

1. Set SRC key on all source directories
2. If step 1 fails, remove xattrs, and fail rename
3. If case2, set xattrs on destination directories
4. If failure in case2, ignore
5. Rename directories (opendir on dst, readdir(ENOEMPTY error), rename 
dst_hashed subvol first, and then rest)
6. If step 5 fails with any error other than ENOTCONN, fail rename, and remove 
xattrs
7. If failure is because of ENOTCONN, proceed with rename and return a success.

Recovery steps (once the brick comes up):

1. On lookup/readdir (NFS requirement?) query for these SRC and DST key.
2. If SRC key is found , validate:
   a. If mtime is less than 5 seconds of the lookup request, then do not heal, 
as rename might be in progress (Can we make this more fool proof?)
   b. If dst does, not exist, proceed
   c. If dst exists, check its key and see if they match. If mismatch, do not 
rename, as it might lead to gfid mis-match.

3. Proceed with checks rename of directories (similar to step 5 of above 
(rename).
4. If successful, remove xattrs, return success.
5. If failure what needs to be done? (other rename's might have succeeded, this 
might fail due to ENOTEMPTY(even due to race)


As for subvol down, we can't guarantee in the scenarios of brick going down 
after stage 1(setxattr).

Brick going down before start of subvolume: We do not allow rename to progress 
anywhere.

If a brick goes down after setxattr, if it has files, or files are created 
after its up (possible race), then we cant recover.


With regards,
Shishir