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On 05/10/2018 06:29 PM, Anatoli wrote:<br>
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<div style="font-size:10.0pt;font-family:Verdana,Arial">Actually,
mysqldump performs a lock on the records it's dumping. If its
for a MyISAM db, the entire table is locked. If it's for
InnoDB and similar, an internal snapshot is created and only
the records the dump is reading are unavailable for writing.<br>
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Mysql provides table consistency by locking but that is only table
consistency.<br>
Multiple updates across multiple tables could easily result in an
inconsistent database even though the tables are individually
consistent.<br>
With some tables that take hours to dump locking the tables is
problematic.<br>
This is why some people use LVM snapshots combined with "FLUSH
TABLES WITH READ LOCK".<br>
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<div style="font-size:10.0pt;font-family:Verdana,Arial"> <br>
Cyrus could also implement a per-user lock, but in reality it
doesn't need that complex syncro mechanisms, a simple global
write lock would be enough (reading would not be affected, son
only I, not I/O, and not to stop it but just to suspend).
After all, the *write* lock would last only a second or so,
the fs snapshots are almost instantaneous. If you can't
tolerate a 1 second delay for writing in Cyrus, you are
probably not a SME.<br>
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The trouble is that read operations can alter a files state so it
may not be just a simple matter of a write lock.<br>
If the mailboxes are on something like EXT4 you can do an LVM
snapshot bacause of the built in auto checkpointing and for
something like xfs there is freeze.<br>
<br>
Cyrus has multiple databases that would also need to be frozen and
flushed before the snapshot is taken.<br>
If you spread your mailstorage and metadata storage over multiple
file systems trying to co-ordinate snapshots becomes more complex.<br>
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<div style="font-size:10.0pt;font-family:Verdana,Arial"> <br>
And you don't need to hold the data to transfer it. You can
dump it directly to a nfs share or pass it as stdout to ssh: <font
face="Courier New">mysqldump | xz -9 | ssh remote_server
"cat > /bkp/</font><font face="Courier New"><font
face="Courier New">`date +%y%m%d_%H%M`.sql.xz</font>"</font>.
With a couple of pipes more you can encrypt the data on the
fly so it's secure to store it in a cheap VPS overseas... or
you could upload it to dropbox.<br>
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<br>
There may be an argument that could be made for 2 backup stratagies.<br>
1) where the mailstoreage and metadata can exist on a single volume
and flushing the various databases is a short duration event. Then
an LVM snapshot could be used <br>
2) for distribted large scale mail systems where only an online live
backup system can be used.<br>
<br>
Backup for 100 users has different requirements than backup for
100000 users so why not support a few different backup strategies.<br>
<br>
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1.0pt;padding:3.0pt 0cm 0cm
0cm;font-size:10.0pt;font-family:"Tahoma","sans-serif""><b>From:</b>
Jason L Tibbitts Iii<br>
<b>Sent:</b> Thursday, May 10, 2018 18:41<br>
<b>To:</b> Anatoli<br>
<b>Cc:</b> Info-cyrus<br>
<b>Subject:</b> Re: Backup methods<br>
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<pre wrap="">"A" == Anatoli <a class="moz-txt-link-rfc2396E" href="mailto:me@anatoli.ws" moz-do-not-send="true"><me@anatoli.ws></a> writes:
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<pre wrap="">A> What about mysqldump > dump.sql, then mysql < dump.sql? Also a wrong
A> way and didn't have to be implemented?
No, that's exactly my point. Thanks for making it for me! The analog
to the way you indicated that you would like it to work would be having
the mysql server stop IO so that you can take a filesystem snapshot
while the database is in a consistent state. But instead, the database
(like cyrus) implements a backup method which you can use to extract the
data. And it also requires disk space to hold the backup until you can
transfer it to your backup medium.
- J<
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