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Re: [PATCH] Introduce per-instance filesystem UUIDs

From: Ivan Zhakov <ivan_at_visualsvn.com>
Date: Sun, 10 Aug 2014 14:29:58 +0400

On 8 August 2014 21:24, Evgeny Kotkov <evgeny.kotkov_at_visualsvn.com> wrote:
> Hi,
> I would like to propose a patch for the problem discussed in
> http://svn.haxx.se/dev/archive-2014-04/0245.shtml
> Please see the details below.
> Log message:
> [[[
> Avoid shared data clashes between repositories duplicated using 'hotcopy',
> see http://svn.haxx.se/dev/archive-2014-04/0245.shtml
> This is not a "scalability issue" (as stated in the referenced thread), but
> rather a full-fledged problem. We have an ability to share data between
> different objects pointing to same filesystems. The sharing works within
> a single process boundary; currently we share locks (svn_mutex__t objects)
> and certain transaction data. Accessing shared data requires some sort of
> a key and currently we use a filesystem UUID for this purpose. However,
> this is *not* good enough for at least a couple of scenarios.
> Filesystem UUIDs aren't really unique for every filesystem an end user might
> have, because they get duplicated during hotcopy or naive copy (copy-paste).
> Whenever we have two filesystems with the same UUIDs open within a single
> process, the shared data starts clashing and things can get pretty ugly.
> For example, one can experience random errors with parallel commits to two
> repositories with the same UUID (hosted by Apache HTTP Server). Another
> example was recently mitigated by http://svn.apache.org/r1589653 — we did
> encounter a deadlock within nested 'svnadmin freeze' commands executed for
> two repositories with the same UUID.
> Errors that I witnessed include (but might not be limited to):
> - Cannot write to the prototype revision file of transaction '392-ax'
> because a previous representation is currently being written by this
> process (SVN_ERR_FS_CORRUPT)
> - Can't unlock unknown transaction '392-ax' (SVN_ERR_FS_CORRUPT)
> - Recursive locks are not supported (SVN_ERR_RECURSIVE_LOCK)
> # This used to be deadlock prior to http://svn.apache.org/r1591919
> Fix the issue by introducing a concept of "instance UUIDs" on the FS layer.
> Basically, this gives us an ability to distinguish filesystem duplicates or
> near-duplicates produced via our API (svn_fs_hotcopy3(), to be exact). We
> can now have different filesystems with the same "original" UUID, but with
> different instance UUIDs. With this concept, it is rather easy to get rid
> of the shared data clashes described above.
> * subversion/libsvn_fs_fs/fs.h
> (fs_fs_data_t.instance_uuid): New.
> * subversion/libsvn_fs_fs/fs_fs.c
> (read_format, svn_fs_fs__write_format): Support the new 'instance-uuid'
> format option.
> (svn_fs_fs__open): Initialize the instance UUID when it is present and
> fallback to the original UUID of a filesystem whenever the instance
> UUID is absent.
> (upgrade_body, svn_fs_fs__create): Generate a new instance UUID when
> necessary.
> * subversion/libsvn_fs_fs/hotcopy.c
> (hotcopy_create_empty_dest): Unconditionally generate a new instance UUID.
> * subversion/libsvn_fs_fs/fs.c
> (fs_serialized_init): Use a combination of two UUIDs as the shared data
> key (see the huge comment block about why we concatenate them).
> * subversion/tests/cmdline/svnadmin_tests.py
> (check_hotcopy_fsfs_fsx): Allow different 'instance-uuid' format options
> when comparing the db/format contents.
> (freeze_freeze): Do not change UUID of hotcopy for new formats supporting
> instance UUIDs. For new formats, 'svnadmin freeze A (svnadmin freeze B)'
> should not deadlock or error out with SVN_ERR_RECURSIVE_LOCK even if 'A'
> and 'B' share the same UUID.
> * subversion/tests/libsvn_fs_fs/fs-fs-pack-test.c
> (write_format): Rework this helper. Before this changeset, we were always
> rewriting 'db/format' contents with predefined values. What we really
> want here is to switch the given filesystem to a sharded layout with a
> specific number of revisions per shard. We might as well achieve this
> by patching the 'layout' format option and doing so would not somehow
> affect the new 'instance-uuid' format option. Implement this logic,
> rename the function into ...
> (patch_format): ...this, and, finally ...
> (create_packed_filesystem): ...update the only caller.
> ]]]
> Are there any objections to this change?
Hi Evgeny,

I've reviewed your patch and I'm +1 to commit: it is clear and fixes
one of fundamental FSFS problems. The most complicated part of the
patch is test suite changes which is good sign :)

Ivan Zhakov
CTO | VisualSVN | http://www.visualsvn.com
Received on 2014-08-10 12:30:48 CEST

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