[Dovecot] prevent dovecot from modifying maildirs
Hi all,
Is there any possibility to ask dovecot to write nothing to disk, something like a "read only" state, without shutting it down? I have set up a ZFS+postfix+dovecot mail server, and I need both postfix (postsuper -h ALL) and dovecot (¿?) to stop modifying the disk to create the snapshot to be saved.
Does anybody has any idea on how to achieve this?
Regards!
FelixOn 2013-08-09 5:44 AM, Felix Rubio Dalmau <felixrubiodalmau@gmail.com> wrote:
Is there any possibility to ask dovecot to write nothing to disk, something like a "read only" state, without shutting it down? I have set up a ZFS+postfix+dovecot mail server, and I need both postfix (postsuper -h ALL) and dovecot (¿?) to stop modifying the disk to create the snapshot to be saved.
???
That is supposed to be (haven't used it yet) one of ZFS's strong points.
You don't need to worry about this, it is all handled by the OS.
If you think you do need to worry about it, then you'll have to come up with a very good reason why.
--
Best regards,
*/Charles/*
On 2013-08-09 11:44, Felix Rubio Dalmau wrote:
Is there any possibility to ask dovecot to write nothing to disk, something like a "read only" state, without shutting it down? I have set up a ZFS+postfix+dovecot mail server, and I need both postfix (postsuper -h ALL) and dovecot (¿?) to stop modifying the disk to create the snapshot to be saved.
Does anybody has any idea on how to achieve this?
As mentioned before, that's done by ZFS. Just create a snapshot using your ZFS functionality (zfs snapshot <name>).
Regards, Michael
On 2013-08-09 8:04 AM, Michael Grimm <trashcan@odo.in-berlin.de> wrote:
On 2013-08-09 11:44, Felix Rubio Dalmau wrote:
Is there any possibility to ask dovecot to write nothing to disk, something like a "read only" state, without shutting it down? I have set up a ZFS+postfix+dovecot mail server, and I need both postfix (postsuper -h ALL) and dovecot (¿?) to stop modifying the disk to create the snapshot to be saved.
Does anybody has any idea on how to achieve this?
As mentioned before, that's done by ZFS. Just create a snapshot using your ZFS functionality (zfs snapshot <name>).
And the same goes for LVM snapshots.
The only thing you might want/need to worry about is SQL databases... for these I always do periodic dumps (how often dicatated by how long the dumps take vs how often the data changes vs how critical the data is).
--
Best regards,
*/Charles/*
Actually the problem is that the snapshot is instantaneous, so if some commands are being run at the time of the snapshot I could get a snapshot of a non-consistent FS, right?
Then, the single solution goes through just shuting down dovecot (and other services, like apache) the seconds required to perform the snapshot, whilst holding the postfix queues to ensure not loosing any mail.
Does anybody have a better approach?
Regards! Felix
On Friday 09 August 2013 08:19:59 Charles Marcus wrote:
On 2013-08-09 8:04 AM, Michael Grimm <trashcan@odo.in-berlin.de> wrote:
On 2013-08-09 11:44, Felix Rubio Dalmau wrote:
Is there any possibility to ask dovecot to write nothing to disk, something like a "read only" state, without shutting it down? I have set up a ZFS+postfix+dovecot mail server, and I need both postfix (postsuper -h ALL) and dovecot (¿?) to stop modifying the disk to create the snapshot to be saved.
Does anybody has any idea on how to achieve this?
As mentioned before, that's done by ZFS. Just create a snapshot using your ZFS functionality (zfs snapshot <name>).
And the same goes for LVM snapshots.
The only thing you might want/need to worry about is SQL databases... for these I always do periodic dumps (how often dicatated by how long the dumps take vs how often the data changes vs how critical the data is).
On 2013-08-09 1:43 PM, Felix Rubio Dalmau <felixrubiodalmau@gmail.com> wrote:
Actually the problem is that the snapshot is instantaneous, so if some commands are being run at the time of the snapshot I could get a snapshot of a non-consistent FS, right?
No, that is not right. If it was it would totally defeat the entire purpose of snapshots (which *is* to get a consistent view of the filesystem).
Then, the single solution goes through just shuting down dovecot (and other services, like apache) the seconds required to perform the snapshot, whilst holding the postfix queues to ensure not loosing any mail.
Does anybody have a better approach?
Yes - read up on how snapshots work (in your case ZFS snapshots) and stop wasting brain cycles worrying about a non existent problem.
As I said earlier, the only exception to this consistency question is with SQL databases.
--
Best regards,
*/Charles /*
On 09.08.2013, at 19:43, Felix Rubio Dalmau <felixrubiodalmau@gmail.com> wrote:
Actually the problem is that the snapshot is instantaneous, so if some commands are being run at the time of the snapshot I could get a snapshot of a non-consistent FS, right?
As mentioned before: Wrong. Have a look at http://en.wikipedia.org/wiki/Zfs#Snapshots_and_clones and http://en.wikipedia.org/wiki/Snapshot_(computer_storage) and alike.
Regards, Michael
Michael, Charles,
thank you for your comments. I had already seen the wikipedia entries about ZFS, but when I have performed my tests, consisting in: generate a program that writes a 100 MB file to disk and perform the snapshot during the file is being written. Obviously the snapshot shows a view of the file with the information and size it had at the time of the snapshot... so I get a portion of it.
In order to save space I prefer no to have such half-files (mails) , and it is for this reason that I am asking if there is any way to prevent this.
I'm not an expert in field of file systems, so I can be asking very naive questions... but I appreciate your comments on this topic.
Thank you
FelixOn Friday 09 August 2013 20:39:31 Michael Grimm wrote:
On 09.08.2013, at 19:43, Felix Rubio Dalmau <felixrubiodalmau@gmail.com> wrote:
Actually the problem is that the snapshot is instantaneous, so if some commands are being run at the time of the snapshot I could get a snapshot of a non-consistent FS, right?
As mentioned before: Wrong. Have a look at http://en.wikipedia.org/wiki/Zfs#Snapshots_and_clones and http://en.wikipedia.org/wiki/Snapshot_(computer_storage) and alike.
Regards, Michael
On Fri, 2013-08-09 at 21:12 +0200, Felix Rubio Dalmau wrote:
Michael, Charles,
thank you for your comments. I had already seen the wikipedia entries about ZFS, but when I have performed my tests, consisting in: generate a program that writes a 100 MB file to disk and perform the snapshot during the file is being written. Obviously the snapshot shows a view of the file with the information and size it had at the time of the snapshot... so I get a portion of it.
In order to save space I prefer no to have such half-files (mails) , and it is for this reason that I am asking if there is any way to prevent this.
If you don't disable mail_fsync, you won't get partially written mails. And I'm not sure if you get them even if you do disable mail_fsync. The mails are first written to temporary files, fsynced, then renamed to the final name. So you might end up with some temporary files, but they won't be visible and they'll be deleted automatically.
The above applies to all mailbox formats except mbox.
Great!
Thank you very much, Timo!
FelixOn Sunday 11 August 2013 22:49:43 Timo Sirainen wrote:
On Fri, 2013-08-09 at 21:12 +0200, Felix Rubio Dalmau wrote:
Michael, Charles,
thank you for your comments. I had already seen the wikipedia entries about ZFS, but when I have performed my tests, consisting in: generate a program that writes a 100 MB file to disk and perform the snapshot during the file is being written. Obviously the snapshot shows a view of the file with the information and size it had at the time of the snapshot... so I get a portion of it.
In order to save space I prefer no to have such half-files (mails) , and it is for this reason that I am asking if there is any way to prevent this.
If you don't disable mail_fsync, you won't get partially written mails. And I'm not sure if you get them even if you do disable mail_fsync. The mails are first written to temporary files, fsynced, then renamed to the final name. So you might end up with some temporary files, but they won't be visible and they'll be deleted automatically.
The above applies to all mailbox formats except mbox.
participants (5)
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Benny Pedersen
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Charles Marcus
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Felix Rubio Dalmau
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Michael Grimm
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Timo Sirainen