what is SNAPSHOT ?

[andrea_53]

Hi people,
I would like to get some information about SNAPSHOT. I think that I will have to use it to make some data copy. Can someone give me documentation about it?
Thanks very much

[PIYUSH D. PATEL]

Hi,

A snapshot is not a 'real' filesystem morelike
it points to the files in your original filesystem that have NOT modified during the
backup process. The files that HAVE modified
in your original filesystem during the backup
will be copied to your snap filesystem so that
you have a true to time backup.

The only files in your snap filesystem are
those files modified during the backup.
If all files change during the backup window
your snaplv needs to be as big as the original
lv. Make sure that you are not leaving your
snap lv mounted/configured after completing
the backup.

use lvremove to get rid of it when your backup
has sucessfully completed. Otherwise choose
a greater lvsnap size to avoid any problem.

Piyush

[PIYUSH D. PATEL]

Hi Andrea,

Check this site

http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B7961-90026/B7961-90026_top.html&con=/hpux/onlinedocs/B7961-90026/00/01/101-con.html&toc=/hpux/onlinedocs/B7961-90026/00/01/101-toc.html&searchterms=SNAPSHOT&queryid=20020529-082448

HTH,
Piyush

[A. Clay Stephenson]

The answer you received is not quite correct. The snapshoting is not done on a file-by-file basis but rather a block-by-block basis. When the snapshot is first made, you specify a snapshot buffer (a block device) that is used to store changes. A changemap is used to determine which blocks have been changed. For example, let's say that you are reading block 500 of the original filesystem as a snapshot. The changemap is examined to see if that block has been changed. If not the ORIGINAL filesystem block is read otherwise a block in the snapshot buffer is substituted. When block 500 is updated for the FIRST time, a copy of the original contents are moved to the snapshot buffer BEFORE the actual write to the original filesystem block occurs. Because this is done at the block level rather than an entire file, only a small amount of extra overhead and snapshot buffer space is required.
That also allows the snapshots to be created in a few seconds rather than having to copy the entire filesystem to a safe place. In addition to the contents of the original block 500, there is also a copy of the original file's inode data (mtime, mode, etc.). Files that are created in the original filesystem after the snapshot are ignored; files removed from the original filesystem must be copied to the snapshot buffer.

Moreover, you don't use lvremove to get rid of a snapshot; you simply do a umount of the snapshot filesystem. Typically, a snapshot buffer that is about 10-20% the size of the original filesystem will suffice for all but the busiest filesystems given a reasonably fast backup device.

[andrea_53]

Thank you very much but I need more documentation if it is possible.
I have VERITAS on Superdome, there is a Snapshot graphic interface?
I wonder it because I read it:

To create a snapshot copy of file system data


Step 1. Select the file system to be backed up.

Step 2. Choose File Systems > Snapshot (Selected menu) or File System Snapshot (Command Launcher).

Step 3. Complete the File System Snapshot dialog box as follows:

ecc

Where do I have to select the file system to be backed up?

Thank you

[Jeff Schussele]

Hi andrea,

Here's a few links to snapshot docs

http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B3929-90011/B3929-90011_top.html&con=/hpux/onlinedocs/B3929-90011/00/00/39-con.html&toc=/hpux/onlinedocs/B3929-90011/00/00/39-toc.html&searchterms=snapshot%7cfilesystem&queryid=20020529-091131

http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B3929-90011/B3929-90011_top.html&con=/hpux/onlinedocs/B3929-90011/00/00/40-con.html&toc=/hpux/onlinedocs/B3929-90011/00/00/40-toc.html&searchterms=snapshot%7cfilesystem&queryid=20020529-091131

http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B3929-90011/B3929-90011_top.html&con=/hpux/onlinedocs/B3929-90011/00/00/41-con.html&toc=/hpux/onlinedocs/B3929-90011/00/00/41-toc.html&searchterms=snapshot%7cfilesystem&queryid=20020529-091131

These deal with the topic in general, as backup & performance respectively.

HTH,
Jeff

[A. Clay Stephenson · Accepted solution]

Actually, you man not be referring to a snapshot filesystem (which is Veritas - vxfs) but rather to a snapshot volume (VxVM) which is also Veritas - Veritas Volume Manager). That also allows you to snapshot. If that is the case, see this link:
http://docs.hp.com/hpux/onlinedocs/B7961-90002/B7961-90002.html