[linux-lvm] Snapshot sizes

[linux-lvm] Snapshot sizes

[Gashaw Teshome]

I would like to use snapshotting to backup an LVM-based machine.  The files 
that I want to backup total about 20G, while LV is about 100G.  What size do 
I need to make the snapshot volume to allow for this much data?

Please let me know if I need to add any more information to make myself 
clearer.
-- 
Gashaw Teshome
System Administrator
Neopets.com

Re: [linux-lvm] Snapshot sizes

[AJ Lewis]

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On Wed, Aug 29, 2001 at 03:05:30PM -0700, Gashaw Teshome wrote:
> I would like to use snapshotting to backup an LVM-based machine.  The files 
> that I want to backup total about 20G, while LV is about 100G.  What size do 
> I need to make the snapshot volume to allow for this much data?
> 
> Please let me know if I need to add any more information to make myself 
> clearer.

You need enough room in the snapshot to store all the changes written to the
disk from the time you take the snapshot to the time you finish backing it
up.  What happens is that when there is a write pending to the original LV,
we first copy the original data to the snapshot LV, then write the new data
to the original LV.  The snapshot only stores the changed data.  It
redirects any queries of data that hasn't changed back to the data stored on
the original LV.  Make sense?

There is more detailed information at:
http://tech.sistina.com/lvm/doc/lvm_howto/Taking_a_backup_using_sna.html#sec:Snapshots_Backup

Regards,
-- 
AJ Lewis
Sistina Software Inc.                  Voice:  612-638-0500
1313 5th St SE, Suite 111              Fax:    612-638-0500
Minneapolis, MN 55414                  E-Mail: lewis@sistina.com
http://www.sistina.com

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 (Unfortunately, the PKS-type keyservers do not work with multiple sub-keys)

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Re: [linux-lvm] Snapshot sizes

[Gashaw Teshome]

> Make sense?

I  think so.  So, if I create a LV of (let's say) 100M, take a snapshot and 
then do my backup, as long as no changes have occured in excess of that 100M
I'll be fine.  The total size of the files on the original LV doesn't matter 
as long as I allocate enough space for the snapshot LV.

Do I have it right?

On Wednesday 29 August 2001 03:31 pm, AJ Lewis wrote:
> You need enough room in the snapshot to store all the changes written to
> the
> disk from the time you take the snapshot to the time you finish backing
> it
> up.  What happens is that when there is a write pending to the original
> LV,
> we first copy the original data to the snapshot LV, then write the new
> data
> to the original LV.  The snapshot only stores the changed data.  It
> redirects any queries of data that hasn't changed back to the data
> stored on the original LV.  Make sense?
>
> There is more detailed information at:
> http://tech.sistina.com/lvm/doc/lvm_howto/Taking_a_backup_using_sna.html
> #sec:Snapshots_Backup
>
> Regards,

-- 
Gashaw Teshome
System Administrator
Neopets.com

Re: [linux-lvm] Snapshot sizes

[AJ Lewis]

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On Wed, Aug 29, 2001 at 05:19:10PM -0700, Gashaw Teshome wrote:
> I  think so.  So, if I create a LV of (let's say) 100M, take a snapshot and 
> then do my backup, as long as no changes have occured in excess of that 100M
> I'll be fine.  The total size of the files on the original LV doesn't matter
> as long as I allocate enough space for the snapshot LV.
>
> Do I have it right?

Yep.  Seems like it.

Regards,
-- 
AJ Lewis
Sistina Software Inc.                  Voice:  612-638-0500
1313 5th St SE, Suite 111              Fax:    612-638-0500
Minneapolis, MN 55414                  E-Mail: lewis@sistina.com
http://www.sistina.com

Current GPG fingerprint = 3B5F 6011 5216 76A5 2F6B  52A0 941E 1261 0029 2648
Get my key at: http://people.sistina.com/~lewis/gpgkey
 (Unfortunately, the PKS-type keyservers do not work with multiple sub-keys)

-----Begin Obligatory Humorous Quote----------------------------------------
Mountain Dew... because breakfast is the most important meal of the day.
-----End Obligatory Humorous Quote------------------------------------------

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