Intelligent Caching Of Write Requests; Journaling Write Requests - IBM N Series Hardware Manual

11.4 Intelligent caching of write requests

Caching writes have been 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 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 would be 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 called a
which is a consistent image of the on-disk file system. A consistency point remains
unchanged even as new blocks are being 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 has received since creation of the last consistency point. With this approach, if a
failure occurs, Data ONTAP reverts to the latest consistency point. It then replays the journal
of write requests from NVRAM to bring the system up to date and make sure 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 several 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 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 merely has to log the 8 KB of data along with about 120 bytes of header
information. Therefore, it uses half or a third as much space.
It is common for other vendors to point out that N series storage systems often have far
less NVRAM than competing models. This is because N series storage systems actually
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
completely self-consistent.
Improves response times. Both block-oriented SAN protocols (Fibre Channel protocol,
iSCSI, FCoE) and file-oriented NAS storage protocols (CIFS, NFS) require an
consistency point
Chapter 11. Core technologies
,
151