[gpfsug-discuss] Querying size of snapshots
Marc A Kaplan
makaplan at us.ibm.com
Tue Jan 29 18:24:17 GMT 2019
1. First off, let's RTFM ...
-d Displays the amount of storage that is used by the snapshot.
This operation requires an amount of time that is proportional to the size
of the file system; therefore,
it can take several minutes or even hours on a large and heavily-loaded
file system.
This optional parameter can impact overall system performance. Avoid
running the mmlssnapshot
command with this parameter frequently or during periods of high file
system activity.
SOOOO.. there's that.
2. Next you may ask, HOW is that?
Snapshots are maintained with a "COW" strategy -- They are created
quickly, essentially just making a record that the snapshot was created
and at such and such time -- when the snapshot is the same as the "live"
filesystem...
Then over time, each change to a block of data in live system requires
that a copy is made of the old data block and that is associated with the
most recently created snapshot.... SO, as more and more changes are made
to different blocks over time the snapshot becomes bigger and bigger. How
big? Well it seems the current implementation does not keep a "simple
counter" of the number of blocks -- but rather, a list of the blocks that
were COW'ed.... So when you come and ask "How big"... GPFS has to go
traverse the file sytem metadata and count those COW'ed blocks....
3. So why not keep a counter? Well, it's likely not so simple. For
starters GPFS is typically running concurrently on several or many
nodes... And probably was not deemed worth the effort ..... IF a
convincing case could be made, I'd bet there is a way... to at least keep
approximate numbers, log records, exact updates periodically, etc, etc --
similar to the way space allocation and accounting is done for the live
file system...
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