IBM N Series, Hardware Manual

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Chapter 11. Core technologies
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The goal of WAFL is to write data in full stripes across the storage media. To write the data, it
holds write requests in NVRAM while it chooses the best location for the data. It then
completes RAID calculations, parity calculations, and gathers enough data to write a full
stripe across the entire RAID group. A sample client request is shown in Figure 11-3.
Figure 11-3 High performance NVRAM virtualization
WAFL never holds data longer than 10 seconds before it establishes a CP. At least every 10
seconds, WAFL takes the contents of NVRAM and commits it to disk. When a write request is
committed to a block on disk, WAFL clears it from the journal. On a system that is lightly
loaded, an administrator can see the 10-second CPs happen; every 10 seconds the lights
cascade across the system. Most systems run with a heavier load than that, and CPs happen
at smaller intervals depending on the system load.
NVRAM does not cause a performance bottleneck. The response time of RAM and NVRAM
is measured in microseconds. Disk response times are always in milliseconds and it takes a
few milliseconds for a disk to respond to an I/O. Therefore, disks are always the performance
bottleneck of any storage system because disks are radically slower than any other
component on the system. When a system starts committing back-to-back CPs, the disks are
taking writes as fast as they can. That is a platform limit for that system. To improve
performance when the platform limit is reached, you can spread the traffic across more heads
or upgrade the head to a system with greater capacity. NVRAM can function faster if the disks
can keep up.
For more information about technical details of N series RAID-DP, see IBM System Storage N
Series Implementation of RAID Double Parity for Data Protection, REDP-4169, which is
available at this website:
http://www.redbooks.ibm.com/abstracts/redp4169.html?Open
CLIENT
Continuous Processing of Client Request
Minimum Interruption of processing
(Resumed) Continuous Processing of Client Request
WAFL
Semantic
Layer
Writes
Reads
NVRAM
RAM
(buffer
cache)
WAFL
Block
Allocation
Layer
RAID
Manager
Disk Subsystem
Virtualization
layer
ƒ
Client RAM-to-NVRAM data path
Fast, predictable client response time
ƒ
Permits virtualization layer to optimize physical access (fewer writes, faster writes)
Better disk subsystem throughput