Hi Jay,
You are right that the terminology is confusing.
One way to look at it is to think of two ways of copy-on-write: when you are updating a block of a file you can: 1) Either write the NEW UPDATED block to a new location 2) Or move the OLD block to a new location AND THEN write the new updated block in place of the old one.
Taking a snapshot with both methods involves copying the inode file BUT obviously there could be serious performance impact using the second method (for each update, the write request needs to 'wait' until the old block gets copied to a new location).
Network Appliance's WAFL uses the first method which does not have any impact on performance and is (IMVHO) much more elegant.
Thanks, Eli
______________________ Eli Lopez Systems Engineer Manager Network Appliance, Israel (+972) - 50 - 304 - 733 Eli.Lopez@Netapp.com ______________________
----- Original Message ----- From: Jay Soffian jay@loudcloud.com To: toasters@mathworks.com Sent: Thursday, May 10, 2001 7:00 PM Subject: Re: EMC IP4700 vs NetApp F740
"Ferd" == Ferd Berfl ferd_berfl@yahoo.com writes:
> re: snapshots, I thought that everyone else's snapshot > functionality other than Netapp's must be based on a "copy on > write" method because Netapp has a patent on continuously > writing new blocks.WAFL uses copy-on-write. Here, straight from the horse's mouth:
WAFL uses a copy-on-write technique to minimize the disk space that Snapshots consume.
http://www.netapp.com/tech_library/3002.html
Now, Netapp's implementation may be unique, but to say that WAFL doesn't use copy-on-write and to make a claim like "copy-on-write is slow, Netapp doesn't use it" is bogus.
j.
Jay Soffian jay@loudcloud.com System
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