[Ocfs2-devel] [RFC] ocfs2/dlm: support range lock

[Goldwyn Rodrigues]

Hi Yangwenfang,

I appreciate the effort in this regard.

On 01/26/2015 06:28 AM, yangwenfang wrote:
> What:
> Byte range lock is applied to lock a region of a file to accelerate
> reading/writing concurrently.
>
> Why:		
> Currently ocfs2 does not support byte range lock. Since multiple nodes
> may concurrently update/write at different positions of the same file
> in database workloads, the performance(tpmc) of DB+ocfs2 is much poorer than
> DB+GPFS in running TPCC.
> Aiming at improving the efficiency of parallel accesses to the same file,
> we have implemented a demo of range lock feature which has been supported
> by lustre and GPFS, so that a file can be updated by different nodes in
> the cluster when they are visiting different blocks.
>
> How:
> Key issues in design and implementation:
> 1.In ocfs2, each file only has one lock, which is incapable of telling
> different position.
> One solution is to add a range field (start,end) in a lock. For example:
> -ocfs2_lock_res(N1)	      dlm_lock_resource(Master)	ocfs2_lock_res(N2)
> -ocfs2_res_range_lock (0,9)----dlm_lock(0,9)    N1			
> -				dlm_lock(10,19)  N2<--ocfs2_res_range_lock(10,19)
> -ocfs2_res_range_lock (20,29)---dlm_lock(20,29)  N1			
> -				dlm_lock(30,49)  N2<--ocfs2_res_range_lock(30,49)
> -ocfs2_res_range_lock (50,59)---dlm_lock(50,59)  N1			
> -				dlm_lock(60,69)  N2<--ocfs2_res_range_lock(60,69)
>
> Each lock resource deploys an interval tree to manage the range, which
> supports basic operations like add, delete, insert, find, split and merge.
> The most important issue is to determine the existance of conflicts
> among the ranges. Conflict-free ranges of the same file can be accessed
> concurrently. In the contrary, nodes must wait for the release of a
> conflicted lock before accessing the range of file.
>
> Byte range lock supports split and merge rules: for same level, larger
> scope; different level, write > read(If a node keeps EX lock with
> range(start,end), then it has PR range lock(start,end)).
> For example:
> (1) merge: N1 keeps range lock (0,9)PR and (5,19)PR, the lock is merged into
> (0,19) PR;
> (2) merge: N1 keeps range lock (0,9)PR and (5,19)EX, the merged lock should
> become(0,19) PR, (5,19)EX;
> (3) split: N1 keeps range lock (0,9)PR, N2 tries to lock(0,5) PR, N1 should
> split the lock and keep (6,9)PR.

What is the purpose of doing this kind of merge/split? I assume this 
will be required in case of multiple processes from the same node 
read/write to the file. Would it not be simpler to not merge or split 
and keep separate instances in lock resources? This way you would have 
to do relatively lesser book keeping with respect to comparisons.

Are these numbers in your pseudocode byte ranges? If yes, how do you 
propose multiple writes which lie within a block_size/cluster_size range?


>
> 2.In ocfs2, there are only three types of lock resources: rw, inode and open
> which provide protections to different contents.
> We need to add another lock resource(ip_range_lock_lockres) to protect
> different ranges in IO read/write process.
> For example: buffer read/write.
> (1)ocfs2_file_aio_write	------------->ocfs2_file_aio_write
> 	ocfs2_rw_lock(ex)		ocfs2_rw_lock(pr)
> 					ocfs2_range_lock(start, end, ex)

This does not seem right. ocfs2_rw_lock is meant to serialize writes to 
the same file. Changing it from ex to pr would make the file 
inconsistent for writes to the same file. As Srini proposed, why create 
a new lock instead of adding the feature to rw_lock?

> 	ocfs2_write_begin
> 		ocfs2_inode_lock(ex)    ocfs2_inode_lock(pr)
> 					if append, update to ex;
> (2)ocfs2_file_aio_read---------------> no need to change.
> 	ocfs2_readpage
> 		ocfs2_inode_lock(pr)
> (3)but it is a problem in read_ahead.
> 	ocfs2_readpages------------------>ocfs2_readpages
> 	ocfs2_inode_lock(pr)		ocfs2_inode_lock(pr)
> 					ocfs2_range_lock(start, end, pr)
> 																	
> Limitations based on our assumption:
> 1.Byte range lock is only beneficial for update write.
> 2.Too many locks because of delayed unlock.
> 3.Significant source code modification is necessitated, involving almost the
> whole dlmglue and dlm modules.
>
> As described above, there are also many limitations base on our assumption.
> Many thanks for any advice.
>


-- 
Goldwyn