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Posted to users@subversion.apache.org by Andrew Martin <am...@xes-inc.com> on 2016/12/13 21:17:53 UTC
Backup using ZFS Snapshots
Hello,
I am running a Subversion 1.9.3 server on Ubuntu 16.04. I currently use
svnadmin hotcopy to safely backup the SVN repositories, but since the
repositories are now hosted on a ZFS dataset, I would like to utilize ZFS's
snapshot capabilities to create atomic, point-in-time backups of the
repositories. My plan to do this is as follows:
1. create zfs snapshot
2. clone zfs snapshot and mount at a temporary location
3. run svnadmin hotcopy from the mounted clone to safely create a backup
4. umount and destroy clone
My only concern is if a commit is in-progress when the zfs snapshot occurs,
would svnadmin hotcopy still be able to safely handle creating the backup?
Is this a safe procedure for creating backups?
Thanks,
Andrew Martin
Re: Backup using ZFS Snapshots
Posted by Stefan Sperling <st...@elego.de>.
On Thu, Dec 15, 2016 at 04:43:29PM +0000, Daniel Shahaf wrote:
> Mark Phippard wrote on Thu, Dec 15, 2016 at 09:40:43 -0500:
> > 2. The easier approach is the start-commit hook. Just have a single master
> > hook-script that all repositories are symlinked to. Assuming you do not
> > use this hook for anything else, you can just have it in place with content
> > like:
> >
> > exit 0;
> >
> > Then when you are doing a backup you change it to something like:
> >
> > echo "Server is in read-only mode for backup. It should be available again
> > within N minutes"
> > exit 1;
>
> If you do this, you need to do the check in pre-revprop-change too,
> because revprop changes don't call start-commit; and also in pre-commit,
> in case a commit-in-flight is currently uploading data \u2014 has passed
> start-commit but not yet called pre-commit.
And pre-lock :)
Re: Backup using ZFS Snapshots
Posted by Daniel Shahaf <d....@daniel.shahaf.name>.
Mark Phippard wrote on Thu, Dec 15, 2016 at 09:40:43 -0500:
> 2. The easier approach is the start-commit hook. Just have a single master
> hook-script that all repositories are symlinked to. Assuming you do not
> use this hook for anything else, you can just have it in place with content
> like:
>
> exit 0;
>
> Then when you are doing a backup you change it to something like:
>
> echo "Server is in read-only mode for backup. It should be available again
> within N minutes"
> exit 1;
If you do this, you need to do the check in pre-revprop-change too,
because revprop changes don't call start-commit; and also in pre-commit,
in case a commit-in-flight is currently uploading data \u2014 has passed
start-commit but not yet called pre-commit.
> That said, based on the approach you are taking, I do not think you need to
> do any of this. Your ZFS snapshot of the filesystem can happen while a
> commit is happening since the ultimate backup will be done with an svnadmin
> hotcopy and that command will not care if the snapshot grabbed an
> in-progress commit.
Re: Backup using ZFS Snapshots
Posted by Andrew Martin <am...@xes-inc.com>.
----- Original Message -----
> From: "Mark Phippard" <ma...@gmail.com>
> To: "amartin" <am...@xes-inc.com>
> Cc: "users" <us...@subversion.apache.org>
> Sent: Thursday, December 15, 2016 8:40:43 AM
> Subject: Re: Backup using ZFS Snapshots
> On Thu, Dec 15, 2016 at 9:25 AM, Andrew Martin <am...@xes-inc.com> wrote:
>
>> > If all of your activity happens via Apache there might be easier ways to
>> make
>> > all your repositories read only during a backup window and you can also
>> always
>> > use the start-commit hook as an easy way to make repositories read only.
>>
>> It's tempting to just stop apache during the backup, but I need to
>> continue to
>> provide read-only access during the backup window, so apache needs to stay
>> on.
>>
>
> I was thinking of two approaches:
>
> 1. Have a "read-only" Apache httpd.conf that you swap into place and do a
> graceful restart and then swap back at end. This configuration would use
> some variant of these directives I took from the svnbook:
>
> # Authorization: Authenticated users only for non-read-only
> # (write) operations; allow anonymous reads
> <LimitExcept GET PROPFIND OPTIONS REPORT>
> Require valid-user
> </LimitExcept>
>
> You would use this concept, not this exact configuration. You would want
> basically be configuring the server to only allow read options.
>
>
> 2. The easier approach is the start-commit hook. Just have a single master
> hook-script that all repositories are symlinked to. Assuming you do not
> use this hook for anything else, you can just have it in place with content
> like:
>
> exit 0;
>
> Then when you are doing a backup you change it to something like:
>
> echo "Server is in read-only mode for backup. It should be available again
> within N minutes"
> exit 1;
>
> That said, based on the approach you are taking, I do not think you need to
> do any of this. Your ZFS snapshot of the filesystem can happen while a
> commit is happening since the ultimate backup will be done with an svnadmin
> hotcopy and that command will not care if the snapshot grabbed an
> in-progress commit.
>
Thanks for the clarification - those both seem like good approaches as well,
but like you said the ZFS snapshot + "svnadmin hotcopy" should be sufficient
for my particular use case.
Thanks again for the help!
Andrew
Re: Backup using ZFS Snapshots
Posted by Mark Phippard <ma...@gmail.com>.
On Thu, Dec 15, 2016 at 9:25 AM, Andrew Martin <am...@xes-inc.com> wrote:
> > If all of your activity happens via Apache there might be easier ways to
> make
> > all your repositories read only during a backup window and you can also
> always
> > use the start-commit hook as an easy way to make repositories read only.
>
> It's tempting to just stop apache during the backup, but I need to
> continue to
> provide read-only access during the backup window, so apache needs to stay
> on.
>
I was thinking of two approaches:
1. Have a "read-only" Apache httpd.conf that you swap into place and do a
graceful restart and then swap back at end. This configuration would use
some variant of these directives I took from the svnbook:
# Authorization: Authenticated users only for non-read-only
# (write) operations; allow anonymous reads
<LimitExcept GET PROPFIND OPTIONS REPORT>
Require valid-user
</LimitExcept>
You would use this concept, not this exact configuration. You would want
basically be configuring the server to only allow read options.
2. The easier approach is the start-commit hook. Just have a single master
hook-script that all repositories are symlinked to. Assuming you do not
use this hook for anything else, you can just have it in place with content
like:
exit 0;
Then when you are doing a backup you change it to something like:
echo "Server is in read-only mode for backup. It should be available again
within N minutes"
exit 1;
That said, based on the approach you are taking, I do not think you need to
do any of this. Your ZFS snapshot of the filesystem can happen while a
commit is happening since the ultimate backup will be done with an svnadmin
hotcopy and that command will not care if the snapshot grabbed an
in-progress commit.
--
Thanks
Mark Phippard
http://markphip.blogspot.com/
Re: Backup using ZFS Snapshots
Posted by Andrew Martin <am...@xes-inc.com>.
----- Original Message -----
> From: "Mark Phippard" <ma...@gmail.com>
> To: "amartin" <am...@xes-inc.com>
> Cc: "users" <us...@subversion.apache.org>
> Sent: Tuesday, December 13, 2016 3:57:04 PM
> Subject: Re: Backup using ZFS Snapshots
> On Tue, Dec 13, 2016 at 4:47 PM, Andrew Martin <am...@xes-inc.com> wrote:
>
>>
>>
>> ----- Original Message -----
>> > From: "Mark Phippard" <ma...@gmail.com>
>> > To: "amartin" <am...@xes-inc.com>
>> > Cc: "users" <us...@subversion.apache.org>
>> > Sent: Tuesday, December 13, 2016 3:35:37 PM
>> > Subject: Re: Backup using ZFS Snapshots
>>
>> > On Tue, Dec 13, 2016 at 4:17 PM, Andrew Martin <am...@xes-inc.com>
>> wrote:
>> >
>> >> Hello,
>> >>
>> >> I am running a Subversion 1.9.3 server on Ubuntu 16.04. I currently use
>> >> svnadmin hotcopy to safely backup the SVN repositories, but since the
>> >> repositories are now hosted on a ZFS dataset, I would like to utilize
>> ZFS's
>> >> snapshot capabilities to create atomic, point-in-time backups of the
>> >> repositories. My plan to do this is as follows:
>> >>
>> >> 1. create zfs snapshot
>> >> 2. clone zfs snapshot and mount at a temporary location
>> >> 3. run svnadmin hotcopy from the mounted clone to safely create a backup
>> >> 4. umount and destroy clone
>> >>
>> >> My only concern is if a commit is in-progress when the zfs snapshot
>> occurs,
>> >> would svnadmin hotcopy still be able to safely handle creating the
>> backup?
>> >>
>> >
>> > svnadmin hotcopy will not have a problem with an in-process commit. That
>> > is kind of one of the points of that command.
>> >
>> >
>> >
>> >> Is this a safe procedure for creating backups?
>> >>
>> >
>> > I know very little about ZFS but I do not understand why you would do
>> > this. If it can take snapshots, then why wouldn't you just take a
>> snapshot
>> > of the actual live repository, why would you want to copy it?
>> >
>> > Setting that aside, I think hotcopy is a very good way to do backups ...
>> > but it is not clear what value would come from taking a snapshot of the
>> > backup given that version control history is immutable etc. Also, once
>> you
>> > create the initial hotcopy, you can use the hotcopy --incremental option
>> to
>> > just copy the newer revisions. Do that from a post-commit hook and you
>> can
>> > always have a backup up to the latest commit.
>> >
>> > To me snapshots would seem like a way to do backups without using
>> hotcopy.
>> > I am not sure the value of combining the two ... but it should work
>> > technically if you have "reasons".
>> >
>> > Mark
>>
>> The reason I want to do it this way is that I want to "pull" the backups to
>> the backup server rather than "pushing" them from the Subversion server. In
>> order to do that, I plan on mounting the clone over NFS on the backup
>> server
>> and then using "svnadmin hotcopy" to pull any new data onto the backup
>> server.
>> Yes, I could just mount the live repository, but I have a number of
>> repositories
>> to backup and using the ZFS snapshot ensures that they are all from a
>> single
>> point-in-time.
>>
>> It sounds like "svnadmin hotcopy" will just ignore any in-progress commit,
>> so
>> even if the ZFS snapshot happened in the middle of a commit, it would still
>> result in a consistent backup.
>>
>>
> Yes, hotcopy will leave you with a consistent repository at the state it
> was in when the backup started. Any commits that are in process will not
> be copied/included in the backup. The problem with hotcopy, is that unless
> you craft a method where you can use --incremental, you are copying the
> entire repository every time you do a backup even though only a small
> number of new files may have been created. The SVN repository format is
> write-once. Only new files get added as new revisions get created. So the
> repository can be backed up very efficiently if you just keep capturing the
> new revisions being created.
>
Yes I plan on using --incremental to make the backup very quick. Basically the
steps will be:
1. make snapshot on fileserver
2. clone snapshot to read-only clone, accessible by backup server over NFS
3. mount clone on backup server
4. run svnadmin hotcopy --incremental on clone
5. unmount and delete clone
Since I'll be pointing "svnadmin hotcopy" at the same target directory on
the backup server each time this is run, I should be able to to take advantage
of --incremental to make this fast.
> If all of your activity happens via Apache there might be easier ways to make
> all your repositories read only during a backup window and you can also always
> use the start-commit hook as an easy way to make repositories read only.
It's tempting to just stop apache during the backup, but I need to continue to
provide read-only access during the backup window, so apache needs to stay on. Yes
I could update the authz file and change all entries from rw to r, but that seems
trickier given that I have a large authz file.
Re: Backup using ZFS Snapshots
Posted by Mark Phippard <ma...@gmail.com>.
On Tue, Dec 13, 2016 at 4:47 PM, Andrew Martin <am...@xes-inc.com> wrote:
>
>
> ----- Original Message -----
> > From: "Mark Phippard" <ma...@gmail.com>
> > To: "amartin" <am...@xes-inc.com>
> > Cc: "users" <us...@subversion.apache.org>
> > Sent: Tuesday, December 13, 2016 3:35:37 PM
> > Subject: Re: Backup using ZFS Snapshots
>
> > On Tue, Dec 13, 2016 at 4:17 PM, Andrew Martin <am...@xes-inc.com>
> wrote:
> >
> >> Hello,
> >>
> >> I am running a Subversion 1.9.3 server on Ubuntu 16.04. I currently use
> >> svnadmin hotcopy to safely backup the SVN repositories, but since the
> >> repositories are now hosted on a ZFS dataset, I would like to utilize
> ZFS's
> >> snapshot capabilities to create atomic, point-in-time backups of the
> >> repositories. My plan to do this is as follows:
> >>
> >> 1. create zfs snapshot
> >> 2. clone zfs snapshot and mount at a temporary location
> >> 3. run svnadmin hotcopy from the mounted clone to safely create a backup
> >> 4. umount and destroy clone
> >>
> >> My only concern is if a commit is in-progress when the zfs snapshot
> occurs,
> >> would svnadmin hotcopy still be able to safely handle creating the
> backup?
> >>
> >
> > svnadmin hotcopy will not have a problem with an in-process commit. That
> > is kind of one of the points of that command.
> >
> >
> >
> >> Is this a safe procedure for creating backups?
> >>
> >
> > I know very little about ZFS but I do not understand why you would do
> > this. If it can take snapshots, then why wouldn't you just take a
> snapshot
> > of the actual live repository, why would you want to copy it?
> >
> > Setting that aside, I think hotcopy is a very good way to do backups ...
> > but it is not clear what value would come from taking a snapshot of the
> > backup given that version control history is immutable etc. Also, once
> you
> > create the initial hotcopy, you can use the hotcopy --incremental option
> to
> > just copy the newer revisions. Do that from a post-commit hook and you
> can
> > always have a backup up to the latest commit.
> >
> > To me snapshots would seem like a way to do backups without using
> hotcopy.
> > I am not sure the value of combining the two ... but it should work
> > technically if you have "reasons".
> >
> > Mark
>
> The reason I want to do it this way is that I want to "pull" the backups to
> the backup server rather than "pushing" them from the Subversion server. In
> order to do that, I plan on mounting the clone over NFS on the backup
> server
> and then using "svnadmin hotcopy" to pull any new data onto the backup
> server.
> Yes, I could just mount the live repository, but I have a number of
> repositories
> to backup and using the ZFS snapshot ensures that they are all from a
> single
> point-in-time.
>
> It sounds like "svnadmin hotcopy" will just ignore any in-progress commit,
> so
> even if the ZFS snapshot happened in the middle of a commit, it would still
> result in a consistent backup.
>
>
Yes, hotcopy will leave you with a consistent repository at the state it
was in when the backup started. Any commits that are in process will not
be copied/included in the backup. The problem with hotcopy, is that unless
you craft a method where you can use --incremental, you are copying the
entire repository every time you do a backup even though only a small
number of new files may have been created. The SVN repository format is
write-once. Only new files get added as new revisions get created. So the
repository can be backed up very efficiently if you just keep capturing the
new revisions being created.
FWIW, I am not sure how concerning it really is if you took a snapshot in
the middle of a commit. From what I recall, there is a very tiny race
condition on the file that says what the HEAD revision is -- db/current.
But in the worst case and there is a problem, you can just run svnadmin
recover to reset it. It is not like you will have to deal with total file
corruption or something. There is also a SQLite DB cache now as well which
might add some other edge case problems on a live repository, but there
would be solutions for any problems there as well.
We have used NetApp snapshots to backup repositories for over a decade and
I am not aware of any problems we have ever had when recovering a
repository. I assume ZFS snapshots could work similarly where the entire
FS could be snapshotted and then presumably it has features like NetApp to
synch those snapshots to a DR location.
--
Thanks
Mark Phippard
http://markphip.blogspot.com/
Re: Backup using ZFS Snapshots
Posted by Andrew Martin <am...@xes-inc.com>.
----- Original Message -----
> From: "Mark Phippard" <ma...@gmail.com>
> To: "amartin" <am...@xes-inc.com>
> Cc: "users" <us...@subversion.apache.org>
> Sent: Tuesday, December 13, 2016 3:35:37 PM
> Subject: Re: Backup using ZFS Snapshots
> On Tue, Dec 13, 2016 at 4:17 PM, Andrew Martin <am...@xes-inc.com> wrote:
>
>> Hello,
>>
>> I am running a Subversion 1.9.3 server on Ubuntu 16.04. I currently use
>> svnadmin hotcopy to safely backup the SVN repositories, but since the
>> repositories are now hosted on a ZFS dataset, I would like to utilize ZFS's
>> snapshot capabilities to create atomic, point-in-time backups of the
>> repositories. My plan to do this is as follows:
>>
>> 1. create zfs snapshot
>> 2. clone zfs snapshot and mount at a temporary location
>> 3. run svnadmin hotcopy from the mounted clone to safely create a backup
>> 4. umount and destroy clone
>>
>> My only concern is if a commit is in-progress when the zfs snapshot occurs,
>> would svnadmin hotcopy still be able to safely handle creating the backup?
>>
>
> svnadmin hotcopy will not have a problem with an in-process commit. That
> is kind of one of the points of that command.
>
>
>
>> Is this a safe procedure for creating backups?
>>
>
> I know very little about ZFS but I do not understand why you would do
> this. If it can take snapshots, then why wouldn't you just take a snapshot
> of the actual live repository, why would you want to copy it?
>
> Setting that aside, I think hotcopy is a very good way to do backups ...
> but it is not clear what value would come from taking a snapshot of the
> backup given that version control history is immutable etc. Also, once you
> create the initial hotcopy, you can use the hotcopy --incremental option to
> just copy the newer revisions. Do that from a post-commit hook and you can
> always have a backup up to the latest commit.
>
> To me snapshots would seem like a way to do backups without using hotcopy.
> I am not sure the value of combining the two ... but it should work
> technically if you have "reasons".
>
> Mark
The reason I want to do it this way is that I want to "pull" the backups to
the backup server rather than "pushing" them from the Subversion server. In
order to do that, I plan on mounting the clone over NFS on the backup server
and then using "svnadmin hotcopy" to pull any new data onto the backup server.
Yes, I could just mount the live repository, but I have a number of repositories
to backup and using the ZFS snapshot ensures that they are all from a single
point-in-time.
It sounds like "svnadmin hotcopy" will just ignore any in-progress commit, so
even if the ZFS snapshot happened in the middle of a commit, it would still
result in a consistent backup.
Thanks,
Andrew
Re: Backup using ZFS Snapshots
Posted by Mark Phippard <ma...@gmail.com>.
On Tue, Dec 13, 2016 at 4:17 PM, Andrew Martin <am...@xes-inc.com> wrote:
> Hello,
>
> I am running a Subversion 1.9.3 server on Ubuntu 16.04. I currently use
> svnadmin hotcopy to safely backup the SVN repositories, but since the
> repositories are now hosted on a ZFS dataset, I would like to utilize ZFS's
> snapshot capabilities to create atomic, point-in-time backups of the
> repositories. My plan to do this is as follows:
>
> 1. create zfs snapshot
> 2. clone zfs snapshot and mount at a temporary location
> 3. run svnadmin hotcopy from the mounted clone to safely create a backup
> 4. umount and destroy clone
>
> My only concern is if a commit is in-progress when the zfs snapshot occurs,
> would svnadmin hotcopy still be able to safely handle creating the backup?
>
svnadmin hotcopy will not have a problem with an in-process commit. That
is kind of one of the points of that command.
> Is this a safe procedure for creating backups?
>
I know very little about ZFS but I do not understand why you would do
this. If it can take snapshots, then why wouldn't you just take a snapshot
of the actual live repository, why would you want to copy it?
Setting that aside, I think hotcopy is a very good way to do backups ...
but it is not clear what value would come from taking a snapshot of the
backup given that version control history is immutable etc. Also, once you
create the initial hotcopy, you can use the hotcopy --incremental option to
just copy the newer revisions. Do that from a post-commit hook and you can
always have a backup up to the latest commit.
To me snapshots would seem like a way to do backups without using hotcopy.
I am not sure the value of combining the two ... but it should work
technically if you have "reasons".
Mark
Re: Backup using ZFS Snapshots
Posted by Mark Phippard <ma...@gmail.com>.
On Tue, Dec 13, 2016 at 4:44 PM, Andrew Martin <am...@xes-inc.com> wrote:
> ----- Original Message -----
> > From: "Stefan Sperling" <st...@elego.de>
> > To: "amartin" <am...@xes-inc.com>
> > Cc: "users" <us...@subversion.apache.org>
> > Sent: Tuesday, December 13, 2016 3:29:50 PM
> > Subject: Re: Backup using ZFS Snapshots
>
> > On Tue, Dec 13, 2016 at 03:17:53PM -0600, Andrew Martin wrote:
> >> Hello,
> >>
> >> I am running a Subversion 1.9.3 server on Ubuntu 16.04. I currently use
> >> svnadmin hotcopy to safely backup the SVN repositories, but since the
> >> repositories are now hosted on a ZFS dataset, I would like to utilize
> ZFS's
> >> snapshot capabilities to create atomic, point-in-time backups of the
> >> repositories. My plan to do this is as follows:
> >>
> >> 1. create zfs snapshot
> >> 2. clone zfs snapshot and mount at a temporary location
> >> 3. run svnadmin hotcopy from the mounted clone to safely create a backup
> >> 4. umount and destroy clone
> >>
> >> My only concern is if a commit is in-progress when the zfs snapshot
> occurs,
> >> would svnadmin hotcopy still be able to safely handle creating the
> backup?
> >>
> >> Is this a safe procedure for creating backups?
> >
> > You don't need to hotcopy at all if you have filesystem snapshots.
> > Check out 'svnadmin freeze'. It was made for this use case.
>
> Thanks for the quick reply - 'svnadmin freeze' looks useful for this
> purpose.
>
> Is there a way to freeze the repository with a command and then unfreeze it
> with another command? I'd ideally like to run several commands on different
> servers while the repository is frozen.
>
If you can use freeze it is a good solution but it is one repository at a
time. The svnbook gives a good example using rsync and freeze to do
backups: http://svnbook.red-bean.com/en/1.8/svn.ref.svnadmin.c.freeze.html
If all of your activity happens via Apache there might be easier ways to
make all your repositories read only during a backup window and you can
also always use the start-commit hook as an easy way to make repositories
read only.
--
Thanks
Mark Phippard
http://markphip.blogspot.com/
Re: Backup using ZFS Snapshots
Posted by Andrew Martin <am...@xes-inc.com>.
----- Original Message -----
> From: "Stefan Sperling" <st...@elego.de>
> To: "amartin" <am...@xes-inc.com>
> Cc: "users" <us...@subversion.apache.org>
> Sent: Tuesday, December 13, 2016 3:29:50 PM
> Subject: Re: Backup using ZFS Snapshots
> On Tue, Dec 13, 2016 at 03:17:53PM -0600, Andrew Martin wrote:
>> Hello,
>>
>> I am running a Subversion 1.9.3 server on Ubuntu 16.04. I currently use
>> svnadmin hotcopy to safely backup the SVN repositories, but since the
>> repositories are now hosted on a ZFS dataset, I would like to utilize ZFS's
>> snapshot capabilities to create atomic, point-in-time backups of the
>> repositories. My plan to do this is as follows:
>>
>> 1. create zfs snapshot
>> 2. clone zfs snapshot and mount at a temporary location
>> 3. run svnadmin hotcopy from the mounted clone to safely create a backup
>> 4. umount and destroy clone
>>
>> My only concern is if a commit is in-progress when the zfs snapshot occurs,
>> would svnadmin hotcopy still be able to safely handle creating the backup?
>>
>> Is this a safe procedure for creating backups?
>
> You don't need to hotcopy at all if you have filesystem snapshots.
> Check out 'svnadmin freeze'. It was made for this use case.
Thanks for the quick reply - 'svnadmin freeze' looks useful for this purpose.
Is there a way to freeze the repository with a command and then unfreeze it
with another command? I'd ideally like to run several commands on different
servers while the repository is frozen.
Re: Backup using ZFS Snapshots
Posted by Stefan Sperling <st...@elego.de>.
On Tue, Dec 13, 2016 at 03:17:53PM -0600, Andrew Martin wrote:
> Hello,
>
> I am running a Subversion 1.9.3 server on Ubuntu 16.04. I currently use
> svnadmin hotcopy to safely backup the SVN repositories, but since the
> repositories are now hosted on a ZFS dataset, I would like to utilize ZFS's
> snapshot capabilities to create atomic, point-in-time backups of the
> repositories. My plan to do this is as follows:
>
> 1. create zfs snapshot
> 2. clone zfs snapshot and mount at a temporary location
> 3. run svnadmin hotcopy from the mounted clone to safely create a backup
> 4. umount and destroy clone
>
> My only concern is if a commit is in-progress when the zfs snapshot occurs,
> would svnadmin hotcopy still be able to safely handle creating the backup?
>
> Is this a safe procedure for creating backups?
You don't need to hotcopy at all if you have filesystem snapshots.
Check out 'svnadmin freeze'. It was made for this use case.