The use of writable checkpoints has many uses in today's organization. Whether it is used for disaster recovery, application stage testing, or database mining, this technology has extreme value and usability. With the release of DART 5.6, the Celerra now adds the capability to write to what was always a read only checkpoint. This writable checkpoint is nothing more than incremental changes layered on a read only checkpoint notably named a baseline checkpoint. Any changes to the writable ckpt's are saved to the same location as the read only ckpt's, the SavVol. Comparatively, a writable ckpt can be served up to NFS, CIFS and FTP clients over the network just as the  standard ckpt can. Here is a quick visual to explain...

So as with any technology there are inherent limitations, without further a do here are WC's...

• Duck Test rules apply...A writable checkpoint is ultimately a filesystem so don't think it won't count against the overall per Data Mover and per cabinet limitations. Which --if you're wondering-- are 2048 FS's per blade across the Celerra line and 2048 FS's per cabinet for the NX4 and NS20. However, 4096 per cabinet for NS40, NS90 and NSX models. For each primary file system you can have up to 16 writable snapshots.
• Refreshing a writable checkpoint requires a refresh of the baseline, read-only checkpoint
• Finally, here are a few FS operations that are not allowed with writable checkpoints although may be with production file systems:
• CDMS (Celerra Data Migration Service)
• CVFS (Checkpoint Virtual File System)
• Extending file systems, a writable checkpoint takes on the characteristics of the PFS therefore changing that is taboo.
• MirrorView
• SnapSure creates, refreshes, or scheduling are not permitted on a writable checkpoint..get over it..
• SRDF (Symmetrix Remote Data Facility)
• VTLU (Virtual Tape Library Unit) no, no on using a WC as a backup target
• and TimeFinder

<<Next are a few addressable questions that  always came to mind with checkpoints, hopefully they will bring clarity to your situation>>

How do I Size SAVVOL?

With any ckpt on the Celerra, appropriate sizing of the SAVVOL is important but more so when implementing WC's. You must now be able to predict and plan for future writes as this has direct implications on your size requirements. Your crutch, of course, is the automatic SAVVOL extension feature but be prudent, do the math, and plan for what's to come.

To assist you with how SnapSure calculates the SAVVOL, here is a quick rundown of the defaults..

• If PFS > 10 GB, then SavVol = 10 GB.
• If (PFS < 10 GB) and (PFS > 64 MB), then SavVol = PFS size.
• If PFS < 64 MB, then SavVol = 64 MB, which is the minimum SavVol size.

Keep in mind, this sizing only takes into account your baseline checkpoint, don't forget your client writes. Relax..I know you won't..

What performance implications can I expect, I heard there are no write penalties with NetApp FlexClones?

The mechanisms behind snapshotting, right or wrong, take a different form as engineered by EMC and NetApp. Whether your tackling  the " Sequential read after random write" (NetAPP) performance problems of WAFL or the "Copy on First Write" (EMC-RLP) amercement,  you as a customer must decide which penalty is more of an issue. BTW, here is some great info on comparing the two here, read up very interesting stuff.

Ultimately, the performance experienced with the WC will depend largely upon how the production file system is laid out and its SavVol. Generally you can expect the same performance as with a read-only checkpoint as arbitrary as that sounds, but as a side note remember..creating, refreshing, restoring, or deleting a checkpoint can pause writes to the PFS for a few seconds. So plan accordingly for even the most simplest of operations.

To conclude, although writable checkpoints are not the be all do all to application/DR testing, they do provide an effective and storage efficient means to addressing today's business and technical perplexities. For more information, please see the links below. And one more thing...Happy Snapshotting!

--Light Reading--

Using SnapSure on Celerra