DB2Storage Objects 21
Figure7. ImageCopy SHRLEVEL CHANGE
Another option forimage copies is the use of the concurrent copy feature, with or
without SnapShot. Concurrentc opyand SnapShot are described in 9.5.2,
“Concurrent Copy” on page 94. These features allowDB2 to cr eatefull image
copies with only a short time interval of dat a unavailability.This is illust rated in
Figure 26 on page 94. The DB2 RECOVER utility is able to handle these copies.
Table2 on page 21 shows the different options available for an image copy with
and without the concurrent option.
Table2. DB2Im age Copy with and without Concurrent Copy
ImageCopyFailuresDuringRecovery
During a recovery,an image copy may fail(for example, due to an I/O error). In
this case, RECOVERattempts to use the dual image copy, assuming that such a
copy exists.If the copy does not exist or also fails, RECOVERignores the copy if
it is an incremental image copy, and uses the log for recovery. Ift he failingim age
copyis a full copy, RECOVER falls back to anearlier full image copy to complete
the recovery.The fallback has a performance penalty,but it helps to insure
availability.
Because the fallback insuresrec overability,some installations do not generate
dual image copies. These installations preferto run frequent incremental image
copies instead.
Deleting Image Copies and ArchiveLogs
Image copies are required for data integrity.The customer must have procedures
to ensure that image copies are deleted only when they are not required
anymore.M oreover,because image copies and archive logs are used together,
thedeletion of these data sets has to be synchronized. For example, there no use
CONCURRENT TYPE SHRLEVEL
FULL INCR REFERENCE CHANGE
YES YES NO YES YESa,b
a.Short unavailability at data set level
b.Not valid for page size larger than 4K
NO YES YES YES YES
APPLICATIONREAD PROCESSING
IMAGE COPY
APPLICATIONWRITE PROCESSING