near-Continuous V.S. True CDP

The file system maintains a cache of changes made to files in memory.  This is commonly called the System Cache in Windows and buffers/cache in Linux.  More accurately this is a Page Cache.  This in-memory representation of files that is found on all modern operating systems like Windows and Linux is only flushed to disk periodically and this technique is called lazy write.  A great explanation can be found here: [http://en.wikipedia.org/wiki/Page_cache] and a [highly technical explanation is available here|http://read.cs.ucla.edu/111/2006fall/notes/lec15].

The important thing to understand is that a critical part of server performance is that writes to disk don't actually get written to the disk platters except periodically and this period is determined by the operating system.  True CDP applications do not see changes written to the Storage Area Network until the operating system flushes its Disk Cache.  This means the view  of the raw SAN disk is in the average case corrupt and out of date compared to the actual state of the system. 

The only way to release these changes form memory to disk in order to get a consistent state of data on disk is to Flush dirty or changed memory pages to disk.  This can be done manually in Linux using the ["sync" utility|http://linux.die.net/man/8/sync].  There is a utility written by [Mark Russinovich|http://technet.microsoft.com/en-us/sysinternals/default.aspx] for Windows with the same name [available here|http://technet.microsoft.com/en-us/sysinternals/bb897438.aspx] as part of the [MS SysInternals tool kit|http://technet.microsoft.com/en-us/sysinternals/default.aspx].

| {color:#000000}{*}Requires SAN to Work{*}{color} \\ | {color:#000000}{*}Yes must have SAN{*}{color} \\ | {color:#000000}{*}No any block device, real or virtual{*}{color} \\ |
| {color:#000000}{*}File System Consistency{*}{color} \\ | {color:#000000}{*}No{*}{color} | {color:#000000}{*}Yes{*}{color} \\ |
| {color:#000000}{*}Application Consistency{*}{color} \\ | {color:#000000}{*}Some Cases{*}{color} \\ | {color:#000000}{*}Yes{*}{color} |
| {color:#000000}{*}Can Backup Files on a NAS Appliance through CIFS or NFS *{color} \\ | {color:#000000}{*}No{*}{color} | {color:#000000}{*}No{*}{color} |
| {color:#000000}{*}Best Suited{*}{color} \\ | {color:#000000}{*}Streaming Disk I/O{*}{color} \\ | {color:#000000}{*}All File I/O{*}{color} |