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Re: AW: SQLite locking for WCNG on NFS

From: Philip Martin <philip.martin_at_wandisco.com>
Date: Wed, 22 Feb 2012 13:42:24 +0000

Mark Phippard <markphip_at_gmail.com> writes:

> On Wed, Feb 22, 2012 at 4:25 AM, Philip Martin
> <philip.martin_at_wandisco.com> wrote:
>> The user has more information than the application can ever have.  If
>> the user wants to be able to run two subtree updates in parallel then
>> exlusive locking must not be enabled.  If the user wants to run status
>> during an update then exclusive locking must not be enabled.
>
> These scenarios are already changes that SQLite brings us. You cannot
> update two subtrees in parallel today. You cannot run status while an
> update or commit is running. As noted in another thread this week,
> you cannot even run svn log when one of these processes is running.

That's not correct. Start and hold an update:

$ gdb -arg subversion/svn/.libs/lt-svn up wc/A/B/C
(gdb) b update_editor.c:close_directory
(gdb) r
Breakpoint 2 at 0x7ffff78eee2f

Run status:

$ svn st wc
  L wc/A/B
! L wc/A/B/C
! L wc/A/B/C/D

Update a different subtree:

$ svn up wc/X/Y/Z
Updating 'wc/X/Y/Z':
A wc/X/Y/Z/Q
Updated to revision 4.

Resume first update:

(gdb) disable 1
(gdb) c
Continuing.
Updated to revision 4.

There are changes in 1.7 that reduce concurrency:

  - in 1.7 a work-queue item will prevent status running. in 1.6 status
    would run when log files were present.

  - during an SQLite write transaction other SQLite read transactions
    may block and even timeout, in 1.6 access to non-locked directories
    didn't block.

So concurrency is reduced but some concurrent operation still works in
1.7.

-- 
uberSVN: Apache Subversion Made Easy
http://www.uberSVN.com
Received on 2012-02-22 14:43:02 CET

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