Re: [linux-lvm] RE: [ADMIN] backup/restore - another - Mailing list pgsql-performance

xfs_freeze is a userspace program included in the xfsprogs rpm. It does run
on Redhat 7.3 (the SGI supplied kernels and userspace for RedHat 7.3 are
somewhat dated; I'd suggest patching the 2.4.21 kernel with XFS 1.3.1
patches and upgrading the userspace programs from the SRPMS). Post to the
linux-xfs mailing list if you need further guidance (lots of people seem to
still run XFS on Redhat 7.3).

> -----Original Message-----
> From: CLeon@phs.org [mailto:CLeon@phs.org]
> Sent: Thursday, October 30, 2003 12:28 PM
> To: linux-lvm@sistina.com; tgl@sss.pgh.pa.us
> Cc: threshar@torgo.978.org; josh@agliodbs.com; markw@osdl.org;
> pgsql-performance@postgresql.org; pgsql-admin@postgresql.org
> Subject: Re: [linux-lvm] RE: [ADMIN] [PERFORM] backup/restore
> - another
>
>
> Does xfs_freeze work on red hat 7.3?
>
> Cynthia Leon
>
> -----Original Message-----
> From: Murthy Kambhampaty [mailto:murthy.kambhampaty@goeci.com]
> Sent: Friday, October 17, 2003 11:34 AM
> To: 'Tom Lane'; Murthy Kambhampaty
> Cc: 'Jeff'; Josh Berkus; markw@osdl.org;
> pgsql-performance@postgresql.org; linux-lvm@sistina.com;
> pgsql-admin@postgresql.org
> Subject: [linux-lvm] RE: [ADMIN] [PERFORM] backup/restore - another
> area.
>
>
> Friday, October 17, 2003 12:05, Tom Lane
> [mailto:tgl@sss.pgh.pa.us] wrote:
>
> >Murthy Kambhampaty <murthy.kambhampaty@goeci.com> writes:
> >> ... The script handles situations
> >> where (i) the XFS filesystem containing $PGDATA has an
> >external log and (ii)
> >> the postmaster log ($PGDATA/pg_xlog) is written to a
> >filesystem different
> >> than the one containing the $PGDATA folder.
> >
> >It does?  How exactly can you ensure snapshot consistency between
> >data files and XLOG if they are on different filesystem
>
> Say, you're setup looks something like this:
>
> mount -t xfs /dev/VG1/LV_data /home/pgdata
> mount -t xfs /dev/VG1/LV_xlog /home/pgdata/pg_xlog
>
> When you want to take the filesystem backup, you do:
>
> Step 1:
> xfs_freeze -f /dev/VG1/LV_xlog
> xfs_freeze -f /dev/VG1/LV_data
>     This should finish any checkpoints that were in
> progress, and not
> start any new ones
>     till you unfreeze. (writes to an xfs_frozen filesystem
> wait for the
> xfs_freeze -u,
>     but reads proceed; see text from xfs_freeze manpage in postcript
> below.)
>
>
> Step2:
> create snapshots of /dev/VG1/LV_xlog and /dev/VG1/LV_xlog
>
> Step 3:
> xfs_freeze -u /dev/VG1/LV_data
> xfs_freeze -u /dev/VG1/LV_xlog
>     Unfreezing in this order should assure that checkpoints
> resume where
> they left off, then log writes commence.
>
>
> Step4:
> mount the snapshots taken in Step2 somewhere; e.g. /mnt/snap_data and
> /mnt/snap_xlog. Copy (or rsync or whatever) /mnt/snap_data to
> /mnt/pgbackup/
> and /mnt/snap_xlog to /mnt/pgbackup/pg_xlog. Upon completion,
> /mnt/pgbackup/
> looks to the postmaster like /home/pgdata would if the server
> had crashed at
> the moment that Step1 was initiated. As I understand it,
> during recovery
> (startup) the postmaster will roll the database forward to this point,
> "checkpoint-ing" all the transactions that made it into the
> log before the
> crash.
>
> Step5:
> remove the snapshots created in Step2.
>
> The key is
> (i) xfs_freeze allows you to "quiesce" any filesystem at any
> point in time
> and, if I'm not mistaken, the order (LIFO) in which you
> freeze and unfreeze
> the two filesystems: freeze $PGDATA/pg_xlog then $PGDATA;
> unfreeze $PGDATA
> then $PGDATA/pg_xlog.
> (ii) WAL recovery assures consistency after a (file)sytem crash.
>
> Presently, the test server for my backup scripts is set-up
> this way, and the
> backup works flawlessly, AFAICT. (Note that the backup script starts a
> postmaster on the filesystem copy each time, so you get early
> warning of
> problems. Moreover the data in the "production" and "backup"
> copies are
> tested and found to be identical.
>
> Comments? Any suggestions for additional tests?
>
> Thanks,
>     Murthy
>
> PS: From the xfs_freeze manpage:
> "xfs_freeze suspends and resumes access to an XFS filesystem (see
> xfs(5)).
>
> xfs_freeze halts new access to the filesystem and creates a
> stable image
> on disk. xfs_freeze is intended to be used with volume managers and
> hardware RAID devices that support the creation of snapshots.
>
> The mount-point argument is the pathname of the directory where the
> filesystem is mounted. The filesystem must be mounted to be
> frozen (see
> mount(8)).
>
> The -f flag requests the specified XFS filesystem to be
> frozen from new
> modifications. When this is selected, all ongoing transactions in the
> filesystem are allowed to complete, new write system calls are halted,
> other calls which modify the filesystem are halted, and all
> dirty data,
> metadata, and log information are written to disk. Any process
> attempting to write to the frozen filesystem will block
> waiting for the
> filesystem to be unfrozen.
>
> Note that even after freezing, the on-disk filesystem can contain
> information on files that are still in the process of unlinking. These
> files will not be unlinked until the filesystem is unfrozen or a clean
> mount of the snapshot is complete.
>
> The -u option is used to un-freeze the filesystem and allow operations
> to continue. Any filesystem modifications that were blocked by the
> freeze are unblocked and allowed to complete."
>
> _______________________________________________
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
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