Ed Wilts said:
>Snapshots appear to
be full copies of the file system, whether anything has changed or not.
>It doesn't matter if
you're modifying files, deleting files, or completely overwriting them.
>It's a good thing
you're a backup expert and not claiming to be a NetApp expert :-)
I know quite a bit about NetApp and have
even been accused a time or too of being a NetApp bigot. Want me to give
you a lecture on how WAFL works? ;)
What I’m concerned about is how much
space each snapshot will take up. Let’s cover one extreme.
- Put
10000 1KB files on the filer
- Take
a snapshot.
- Delete
those 10000 files from the filer
- Add
10000 more 1 KB files on the filer
- Take
a snapshot
- Repeat
Each snapshot will take up 10 MB (10,000
KB) of space
Here’s another extreme.
- Put
10000 files on the filer
- Take
a snapshot.
- Modify
each of the 10000 files, with a 1 byte modification each time
- Take
a snapshot
- Repeat
Each snapshot in this scenario will take
up 10 KB (10000 bytes), regardless of the size of the original filer.
If each day he backs up the SQL dump, he
deletes yesterday’s file then makes a new one, he’s behaving like
the first extreme and each snapshot will take up the size of the SQL
dump. If he overwrites the same file every day, he has a better chance of
being closer to the second extreme. But, what I’ve SEEN is that in
the scenario where you are completely overwriting the file each time (as you
would in a SQL dump), it can cause the first extreme and not behave like the
second extreme. It depends on the application and how they lay down the
data. If how they lay down the data makes it look like they’ve just
modified the file, then the filer will act closer to the second extreme.
If the overwriting of the file “scrambles” it in such a way that it
moves all the blocks around, each day’s snapshot will take up the same
space as a full SQL dump. (BTW, the quickest way to guarantee the latter,
in my experience, is to run that SQL dump through compression. Just like
compression messes up dedupe, it also messes up the way NetApp looks at the
file to find differences.)
