[Samba] File permissions
freemyer at NorcrossGroup.com
Tue Aug 20 08:18:01 GMT 2002
Welcome to the nasty world of backups. Everyone thinks of them as being a =
simple thing to control, but in reality they are pretty difficult. I have read =
that 50% of all backups have corrupted data on them for exactly the reasons you =
are mentioning. :<
I had a Server backup fail for this reason at a Major Fortune 500 company a few =
months ago, and the group I was supporting was paying the internal backup team =
thousands of dollars a month to do the backups. :< They were supposed to be =
backup specialists, but were ignoring errors about open files!!!!
Anyway, the standard *NIX statement is that it is the admins job to queise the =
system prior to performing the backup. i.e. Make sure there are no open files.
Traditionally a backup window is defined each night. Then all the server apps =
are shutdown, backups performed, apps restarted.
Some zealous admins use the fuser command to kill any remaining processes using =
a filesystem just prior to backing it up. Then unmount the FS and re-mount it =
read-only for the duration of the backup.
Obviously, today's 7x24 operations cannot handle the multiple hour backup =
windows this requires, so a lot of enterprise class apps have added the ability =
to accept a queise command, you to your backup, then let them go again.
i.e. Oracle does this for sure, but I think a lot of others do it as well.
SInce this is for a 7x24 operation, it is your job to do the backup and get the =
app running again in just a few seconds as opposed to the multiple hours it =
This is where snapshots and clones come into play. Either one can create a =
read-only FS image in just a few seconds. Then you mount and backup the image =
at your leisure.
You mentioned Linux I think, and Linux supports LVM from Sistina which supports =
snapshots. (In theory anyway, I'm trying to get them to work as we speak, and =
they seem broken to me.)
Other *NIX have other software to do this, and most external raid arrays =
support snapshots and clones in hardware.
Deployment and Integration Specialist
Compaq ASE - Tru64 v4, v5
Compaq Master ASE - SAN Architect
The Norcross Group
>> This really has me flabbergasted.
>> cp doesn't seem to care.
>> I just tried tar, and it happily tarred an open file, or at least a file
>> that was being writen to with a piping command.
>> So, how is wholesale file corruption avoided when using tar as a backup
>> program or mv'ing or cp'ing directories as a backup tool?
>> On Tue, Aug 20, 2002 at 12:03:40PM -0400, David Brodbeck wrote:
>> > Most Linux utilities have no real concept of file locking. I think cp
>> > quite happily copy a file that's in the process of being written to.=20
>> > not sure what'd happen if you tried to mv or rm it. mv might be okay,
>> > long as it was to the same filesystem, since the inode would stay the
>> > > -----Original Message-----
>> > > From: Joel Hammer [mailto:Joel at HammersHome.com]
>> > > Sent: Tuesday, August 20, 2002 11:59 AM
>> > > To: David Brodbeck; samba at lists.samba.org
>> > > Subject: Re: [Samba] File permissions
>> > >=20
>> > >=20
>> > > I would be surprised if linux would be so impolite as to=20
>> > > attempt to move a
>> > > file that was being writen to. Would cp or rm be so badly behaved?
>> > > But, I get surprised everyday.
>> > > I guess the only way to be sure is to try it out and see.=20
>> > > Have the cron job
>> > > run every second and start saving some big files. Or, you=20
>> > > might use smbstatus to
>> > > make sure the file wasn't in use before acting on it.
>> > > Joel
>> > > On Tue, Aug 20, 2002 at 11:27:42AM -0400, David Brodbeck wrote:
>> > > > One problem I see with this approach is how do you know the=20
>> > > file is done
>> > > > being written before you move it? You might get unlucky=20
>> > > and move half a
>> > > > file.
>> > > >=20
>> > > > > -----Original Message-----
>> > > > > From: Joel Hammer [mailto:Joel at HammersHome.com]
>> > > > > Sent: Tuesday, August 20, 2002 10:35 AM
>> > > > > To: Scott Mayo; samba at lists.samba.org
>> > > > > Subject: Re: [Samba] File permissions
>> > > > >=20
>> > > > >=20
>> > > > > Another approach.=20
>> > > > >=20
>> > > > > If what you are doing is getting the kids to send in school=20
>> > > > > assignments via
>> > > > > the share, why not write a cron job which monitors that=20
>> > > > > share, say ever 5
>> > > > > secs, and when a file appears on the share, it is removed=20
>> > > and stored
>> > > > > somewhere else.
>> > >=20
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