IBM N Series, Hardware Manual

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Chapter 11. Core technologies
169
11.4 Intelligent caching of write requests
Caching writes were used as a means of accelerating write performance since the earliest
days of storage. The N series uses a highly optimized approach to write caching that
integrates closely with the Data ONTAP operating environment. This approach eliminates the
need for the huge and expensive write caches that are seen on some storage arrays. It
enables the N series to achieve exceptional write performance, even with RAID 6
(double-parity RAID).
11.4.1 Journaling write requests
When any storage system receives a write request, it must commit the data to permanent
storage before the request can be confirmed to the writer. Otherwise, if the storage system
experiences a failure while the data is only in volatile memory, that data is lost. This data loss
can cause the underlying file structures to become corrupted.
Storage system vendors commonly use battery-backed, nonvolatile RAM (NVRAM) to cache
writes and accelerate write performance while providing permanence. This process is used
because writing to memory is much faster than writing to disk. The N series provides NVRAM
in all of its current storage systems. However, the Data ONTAP operating environment uses
NVRAM in a much different manner than typical storage arrays.
Every few seconds, Data ONTAP creates a special Snapshot copy that is called a
consistency
point
, which is a consistent image of the on-disk file system. A consistency point remains
unchanged even as new blocks are written to disk because Data ONTAP does not overwrite
existing disk blocks. The NVRAM is used as a journal of the write requests that Data ONTAP
received since the last consistency point was created. With this approach, Data ONTAP
reverts to the latest consistency point if a failure occurs. It then replays the journal of write
requests from NVRAM to bring the system up to date and ensure the data and metadata on
disk are current.
This is a much different use of NVRAM than that of traditional storage arrays, which cache
writes requests at the disk driver layer. This use offers the following advantages:
򐂰 Requires less NVRAM. Processing a write request and caching the resulting disk writes
generally take much more space in NVRAM than journaling the information that is
required to replay the request. Consider a simple 8 KB NFS write request. Caching the
disk blocks that must be written to satisfy the request requires the following memory:
– 8 KB for the data
– 8 KB for the inode
– For large files, another 8 KB for the indirect block
Data ONTAP must log only the 8 KB of data with approximately 120 bytes of header
information. Therefore, it uses half or a third as much space.
It is common for other vendors to highlight that N series storage systems often have far
less NVRAM than competing models. This is because N series storage systems need less
NVRAM to do the same job because of their unique use of NVRAM.
򐂰 Decreases the criticality of NVRAM. When NVRAM is used as a cache of unwritten disk
blocks, it becomes part of the disk subsystem. A failure can cause significant data
corruption. If something goes wrong with the NVRAM in an N series storage system, a few
write requests might be lost. However, the on-disk image of the file system remains
self-consistent.