LMV Snapshots

[Paul Lussier]

mike ledoux <mwl+gnhlug at alumni.unh.edu> writes:

> The proper way to use an LVM snapshot for backups is:
>
>   a) stop writes to the FS
>   b) take the snapshot
>   c) resume writes to the FS
>   d) mount the snapshot
>   e) make backup
>   f) umount snapshot
>   g) remove snapshot
>
> The "win" here is a few seconds of downtime to take the snapshot
> vs. a few hours of downtime to take a consistent backup without
> snapshots.

Another, slightly more expensive way, but usually preferred by the
paranoid is to use a 3-way mirror.  That procedure would look like:

  a) stop writes to the FS
  b) detach 1/3 of the mirror
  c) resume writes to the FS
  d) mount the mirror
  e) make backup
  f) umount mirror
  g) re-attach the mirror.

The win here, at the expense of it requiring triple the amount of disk
space[1] is that you get a major boost on read performance, and you're
never at risk of losing data, since your data is always mirrored.

klussier at comcast.net writes:

> And, for complete disaster recovery, it's a RAID 10 system ;-)

For a complete disaster recovery, you want spare systems in a rack at
another location to which your offsite backups can be delivered!

And, for those *really* concerned about data loss due to drive
failure, you'd be using RAID 015 or 016 with SCSI or FC drives.[2][3]


[1] Funny how when you're running out of it, "disk space is cheap",
    but when you want to plan for stability, performance, and disaster,
    "it costs too much" :)

[2] I'm wondering how many digits can be added to RAID levels before
    some marketing weenie come up for snazzy and descriptive name for
    it that really doesn't describe at all what's going on, but makes
    you want to buy it anyway :) I envision RAID levels getting to
    look like: RAID 016+501+3+1.  At which point we'll just give up
    and add them all together and call it RAID 521, which is
    completely deceptive :)

[3] The *truly* paranoid completely eschew all this technology and
    write things down on paper in invisible ink in a double-secret
    encryption scheme and lock it all away in their underground bunker
    where we have rations for 100+ years.

    <voice 1> Shhh, Don't *tell* them that!  You're giving away all
              our secrets!

    <voice 2> Oh be quiet, I didn't say anything important.

    <voice 1> Oh yeah, we'll see who's sorry when a million people
              show up at the bunker when Armageddon starts!

    <voice 2> Oh sure!  Start mentioning our secret plans why don't
              you!  Now who's giving away secrets?!

-- 
Seeya,
Paul "Who's finished his "group" meeting now :)
--
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