Sparing in Symmetrix systems
219
Data Integrity, Availability, and Protection
Dynamic sparing with
locally mirrored pairs
When a dynamic spare is invoked for a locally mirrored pair, the
Symmetrix system automatically augments the original mirrored pair
with a dynamic spare volume that joins the mirrored pair as an
additional or (third) mirror shown in Figure 50 on page 219. Data is
copied to the dynamic spare volume from the failing volume. If any
data cannot be copied from the failing volume, it is copied from the
other mirror. The Symmetrix system continues processing I/O
requests with the spare functioning as a mirror and with no
interruption in operation.
The failing disk can then be replaced and resynchronized with the
mirror group. Then, the dynamic spare can be returned to the spare
pool.
Figure 50 Dynamic sparing with locally mirrored pairs
Dynamic sparing with
RAID 5 volumes
RAID 5 is architected around block-based multispindle parity
protection. RAID 5 implements data striping and rotating parity
across all hypervolumes of a RAID 5 device. RAID 5 uses a single
dynamic spare to temporarily replace a failing drive.
The RAID 5 dynamic spare is a part of the RAID group and replaces
the failing member. In particular, if you look at the tracks on the spare
you would see some data tracks and some parity tracks; the entire
layout would be identical to that of the failing member.
Mirrored pair
M1/M2 protected by
dynamic spare DS
M1 M2
M1 M2 M3
M1 M2
M1 M2
M3
M2 failing, dynamic
spare invoked
DS joins 3-way
mirror M1/M2/M3
DS
DS
DS
DS
Failed disk
replaced and new
disk restored as M2
DS returns to
spares pool
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