[Gluster-devel] Fwd: Re: [Gluster-users] Fencing FOPs on data-split-brained files

[Ravishankar N]

-------- Original Message --------

Subject:	Re: [Gluster-users] Fencing FOPs on data-split-brained files
Date:	Tue, 19 Nov 2013 16:03:14 +0530
From:	Ravishankar N <address@hidden>
To:	Anand Avati <address@hidden>
CC:	Gluster Devel <address@hidden>, "address@hidden" <address@hidden>

On 11/16/2013 01:42 AM, Anand Avati wrote:

Ravi,

We should not mix up data and entry operation domains, if a file is in data split brain that should not stop a user from rename/link/unlink operations on the file.

Regarding your concern about complications while healing - we should change our "manual fixing" instructions to:

- go to backend, access through gfid path or normal path

- rmxattr the afr changelogs

- truncate the file to 0 bytes (like "> filename")

Accessing the path through gfid and truncating to 0 bytes addresses your concerns about hardlinks/renames.

Avati

Resending the mail again as there was no response
-Ravi

All,

I have tabulated what operations must/ mustn't be permitted in case of different split brains. Some of the columns are '?' as I am not sure what the expected behaviour should be. Could we have this validated?

File Operation permitted	Type of Split Brain
	Data SB	Metadata SB	Entry SB
			Same entry gfid mismatch SB	Different entries
write	No	Yes (currently no)	No	Yes
read	No	Yes (currently no)	No	Yes
getfattr	Yes	No	No	Yes
lookup	?	?	No	Yes
stat/fstat	?	?	No	Yes
setfattr	Yes	No	No	Yes
touch	Yes	Yes	No	Yes
hard link creation	Yes	Yes	No	Yes
soft link creation	Yes	Yes	Yes	Yes
rename	Yes	Yes	no	Yes
chown	Yes	Yes	Currently No	Yes
chmod	Yes	Yes	Currently No	Yes
unlink	Yes	Yes	Currently No	Yes
readdir	N/A	N/A	?	?

- stat() also reports the file size. If a data split-brained file has different sizes, should stat succeed?
- Likewise if metadata split brain is due to different access permissions, say one brick has file chmod'ed with 777 and the other brick has it with 744, should we allow read/write if the corresponding permission bits are *not* conflciting ? ( as of today they aren't allowed)

Also,In the table above, Entry Split brain has 2 cases-
i) where same entry has different gfids
ii) each brick  has different entries for the same directory (which can cause deleted files to appear in case of conservative merge).
Should we allow readdir in either case?

Thanks,
Ravi

On Wed, Nov 13, 2013 at 3:01 AM, Ravishankar N <address@hidden> wrote:

Hi,

Currenly in glusterfs, when there is a data splt-brain (only) on a file, we disallow the following operations from the mount-point by returning EIO to the application:
- Writes to the file (truncate, dd, echo, cp etc)
- Reads to the file (cat)
- Reading extended attributes (getfattr) [1]

However we do permit the following operations:
-creating hardlinks
-creating symlinks
-mv
-setattr
-chmod
-chown
--touch
-ls
-stat

While it makes sense to allow `ls` and `stat`, is it okay to  add checks in the FOPS to disallow the other operations? Allowing creation of links and changing file attributes only seems to complicate things before the admin can go to the backend bricks and resolve the splitbrain (by deleteing all but the healthy copy of the file including hardlinks). More so if the file is renamed before addressing the split-brain.
Please share your thoughs.

Thanks,
Ravi

[1] http://review.gluster.org/#/c/5988/
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