HP XP P9500 Storage - Manual (Page 13)

Differences in methods for storing snapshots depending on volumes
The method in which you store snapshot data is dependent on the types and RAID levels of P-VOLs.
Use the following list to determine which method to use to store snapshot data:
•
The P-VOL is an internal volume.
If the P-VOL RAID level is RAID 1, use the COW method. ◦
◦ If the P-VOL RAID level is something other than RAID 1, use the CAW method.
•
The P-VOL is a THP volume.
If one of the following applies to the pool, use the CAW method:
◦ The pool is a RAID5 pool
◦ The pool contains volumes of different RAID levels
◦ The pool contains external volumes created using the DKCMAIN microcode program
version 70-02-0X-XX/XX or later
◦ THP and Smart pools
•
If the pool is none of the above, use the COW method. In the above cases, even if the snapshot
data is stored with the CAW method, and if the write pending rate is 60% or more in the
cache memory, use the COW method to store the snapshot data.
For more information about the write pending rate, see the HP XP P9000 Performance for
Open and Mainframe Systems User Guide.
•
The P-VOL is an external volume.
Use the COW method.
CAW storage method workflow
Use the following process to store snapshot data using the CAW method:
1. The host writes data to a P-VOL.
2. The write completion status is returned to the host before the snapshot data is stored.
3. Snapshot data for the P-VOL is stored in the background.
COW storage method workflow
Use the following process to store snapshot data using the COW method:
1. The host writes data to a P-VOL.
2. Snapshot data for the P-VOL is stored.
3. The write completion status is returned to the host.
Restoring Fast Snap pairs
Use FS to overwrite snapshot data in pools to P-VOLs so that the P-VOL content is returned to the
condition it was when you stored the snapshot data. Overwriting snapshot data to P-VOLs restores
FS pairs.
If data is written to a S-VOL, this data (not the snapshot data) is overwritten to the P-VOL when the
FS pair is restored.
If a problem occurs with a P-VOL due to a failure, you can restore the pair so that data in the P-VOL
is returned to a normal condition it was when snapshot data was stored.
For more information about restoring pairs, see “Restoring pairs” (page 48) .
How Fast Snap works 13