it's best described in Timefinder guide for Symmetrix/VMAX (btw, there is no copy on first access in VNX)

http://www.emc.com/collateral/hardware/white-papers/h8264-whats-new-timefinder-emc-symmetrix-vmax-wp.pdf

See below.  Been asked before

https://community.emc.com/message/597902

It says for COFA;

"With the CopyOnAccess, actual copying of the data is deferred until either tracks on the source device are written to or tracks on the target device are read or written.  i.e. after activating the copy session, only those tracks that have been written to the source or written/read from the target will be copied to the target device"

Then COFW also works similarly. Here copy of the original data block (1 x Read I/O) takes place, then this original data block is written to a new location (1 x Write I/O) and then the new data block is written to the location of the original data block (1 x Write I/O).

I am confused.

And how is that a VNX question ?

My fault to say it a VNX question, but basically I wanted to know their difference.

So from

"CopyOnAccess will copy the data if you are simply reading the snapshot (and of course if source is being written to). COFW will only copy the data if source data is being written to"

Can I consider the difference of only "reading" ?

yes

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glen