SUMMARY: Veritas File System Snapshots

first, thanks to Bevan Broun who had the sage advice to rtfm:

      This is all documented in the manual. If the snapshot  file system runs out
      of space is will be disabled and unmounted.

      BB

I do appreciate the thought, but I had actually read the manual and knew that part, I just didn't communicate it effectively (I'm afraid of what might happen if the snapshot runs out of space...  really meant, I'm afraid of running out of space, having the file system remove itself, and my backups failing because I didn't have a static image of the Oracle databaase to restore.)

My bad completely, and I'm glad there are other folks who think that reading the manual is always the best 'first' idea.


second -- to answer my question of how do you find out how much space is being used on the snapshot file system, I found a neat little tool "vxtrace"

# vxtrace oradev-back
<snip snip>
6386 START write vol oradev-back op 0 block 260104 len 16
6386 END write vol oradev-back op 0 block 260104 len 16 time 0
<snip snip>

where "260104" is the number of bytes written to the volume since boot -- which works out nicely to the number of bytes being used on the volume (since the snapshot volume zeros out when it's unmounted...)

you can also use vxstat:

# vxstat -v oradev-back
                        OPERATIONS           BLOCKS        AVG TIME(ms)
TYP NAME              READ     WRITE      READ     WRITE   READ  WRITE 
vol oradev-back        357      7339      5712    457508    0.6    1.5 
# 

where 457508 is the number of blocks written (it seems different than the vxtrace number...  not sure which to trust actually....  but they are both WAY smaller than 1/3 the file system size, so I'm just gonna go with it for now)

thanks again all!

toph


topher originally said:
Does anyone have any experience with Veritas File System Snapshots?

I'm noticing that when you create the snapshot, it assumes the characteristics of the original file system (even if the 'snapshot' volume is 10G and the 'live' file system is 40G, the 'snapshot' reports as 40G)

How do you know how much of your snapshot device is being used, and what happens if it hits 100%?  I don't want to create snapshot volumes that are as large as the original file systems (that would defeat the purpose) -- but I'm afraid of what might happen if the snapshot runs out of space....

anything you've got that will help with this one would be appreciated!

toph
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