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October 27th, 2010 07:00
CopyOnAccess and CopyOnWrite
Hi ,
I like to confim few answers ..
When you create a clone using "Symclone ..Create" then by default it will use CopyOnAccess right ? By default clones using CopyOnAccess and Snaps using CopyOnWrite ?
If snaps used copyonwrite, then why it may cause a performance issues ?
Thanks.
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PedalHarder
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October 27th, 2010 13:00
SNAP CopyOnWrite (COW) is actually asynchronous CopyOnWrite(ACOW).
What this means is that from a host perspective, the write is not held up if i'ts COW. The DMX will do the back-end work to process the COW track without holding up the write IO. The COW process reads the pre-update track from the source device and writes it to the save ares. Then the host write is written to the disk.
We say CopyOnWrite MAY cause performance issues because the asynchronous part of the COW is turned off when the DMX back-end (DA's) are under extremely high load. When ACOW is not active, the host write must wait for the pre-update data on the source disk to be read into cache. From a performance perspective, the write response time goes up to around the time you would expect from a read-miss operation. Note that ACOW is never turned off in a VMAX.
Hope this helps.