M13 Writes-Reads FINAL

7/29/2019 M13 Writes-Reads FINAL

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Write and ReadRequest Processing

Module 13

Data ONTAP® 7.3 Fundamentals



© 2008 NetApp. All rights reserved. 2

Module Objectives

By the end of this module, you should be able to:

Describe how data is written from and read to

a WAFL file system on a volume

Describe the WAFL file system, including

consistency points, RAID management, andstorage levels

Describe how RAID is used to protect disk data

Describe how the WAFL file system processeswrite and read requests




Data ONTAP Simplified

StorageRAIDProtocolsNetwork

Client

Disks

N e t w o r k

Data ONTAP

NVRAM

Client

Client

Client

Client

WAFL ®

Physical

Memory




Write Requests




Write Requests

Write requests are received by Data ONTAP

through multiple interface protocols:

– CIFS

– NFS

– FCP

– iSCSI

– HTTP

– WebDAV Write requests are buffered into:

– System memory

– NVRAM




Write Request Data Flow: Write Buffer

Network

SAN Host

UNIX

Client

WindowsClient

NetworkStack

RS-232

HBA

NIC

Protocols

SAN

Service

NFS

Service

CIFS

Service

Memory Buffer

RAID

Storage

NV

R

A

M

NVRAM Full

WAFL

NVLOGNVLOG

NVLOG

NVLOG

NVLOG


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Consistency Point

A completely self-consistent image of the file

system. Although dynamic, if the file system

were frozen momentarily to capture its

structure –that is a consistency point

A consistency point occurs when designateddata is written to a disk and a new root inode is

determined

A consistency point occurs for multiple

reasons, including the following:

– One bank of the NVRAM card is full

– 10 seconds have elapsed



Consistency Point (Cont.)

During a consistency point, Data ONTAP

flushes writes to disk

– Always writes to new data blocks

– The volume is always consistent on disk

When Data ONTAP flushes memory to disk: – It updates the file system ―atomically,‖ meaning

that the entire write must be completed or the

entire write is rolled back

– This includes all metadata

– After checking, the NVRAM is cleared



WAFL

Write Request Data Flow: WAFL to RAID

Network

SAN Host

UNIX

Client

WindowsClient

NetworkStack

RS-232

HBA

NIC

Protocols

SAN

Service

NFS

Service

CIFS

Service

Memory Buffer

RAID

Storage

NV

R

A

M

NVLOGNVLOG

NVLOG

NVLOG

NVLOG

NVRAM Full



Consistency Point: WAFL to RAID

The RAID layer calculates the parity of thedata:

– To protect it from one or more disk failures

– To protect stripes of data

If a data disk fails, the missing information canbe calculated from parity

The storage system can be configured aseither:

– RAID 4— Allows one disk failure in the RAIDgroup

– RAID-DP— Allows up to two disk failures in theRAID group



Write Request Data Flow: RAID to Storage

Network

SAN Host

UNIX

Client

WindowsClient

NetworkStack

RS-232

HBA

NIC

Protocols

SAN

Service

NFS

Service

CIFS

Service

Memory Buffer

RAID

Storage

NV

R

A

M

NVLOGNVLOG

NVLOG

NVLOG

NVLOG

NVRAM Full

WAFL

4k



Consistency Point: RAID to Storage

Storage layer commits the data and parity to

the physical disks

Two atomic checksums are calculated to verify

that the data is written consistently on the disks

The root inode is updated to point to the newfile inodes on the disk

The NVRAM is flushed and made available

The consistency point now is complete



Write Request Data Flow: Storage Writes

Network

SAN Host

UNIX

Client

WindowsClient

NetworkStack

RS-232

HBA

NIC

Protocols

SAN

Service

NFS

Service

CIFS

Service

Memory Buffer

RAID

Storage

NV

R

A

M

NVLOGNVLOG

NVLOG

NVLOG

NVLOG

NVRAM Full

WAFL



NVRAM

Data ONTAP writes from system memory

– NVRAM is never used for normal write

operations

– NVRAM is backed up with a battery

If a system failure occurs before thecompletion of a consistency point, the data is

written from NVRAM when the system is

brought back online (or by the partner machine

in an active-active environment)



Read Requests



Read Requests

Every time a read request is received, WAFL

does one of the following:

– Reads the data from the system memory, also

known as ―cache‖

– Reads the data from the disks The cache is populated by:

– Data recently written to disk

– Data recently read from disk



Read Request Data Flow: Cache

Network

SAN Host

UNIX

Client

WindowsClient

NetworkStack

RS-232

HBA

NIC

Protocols

SAN

Service

NFS

Service

CIFS

Service

Memory Buffer

RAID

Storage

N

V

R

A

M

NVLOG

NVLOG

NVLOG

NVLOG

NVLOG

NVRAM Full

WAFL

Cache



Read Request Data Flow: Read from Disk

Network

SAN Host

UNIX

Client

WindowsClient

NetworkStack

RS-232

HBA

NIC

Protocols

SAN

Service

NFS

Service

CIFS

Service

Memory Buffer

RAID

Storage

N

V

R

A

M

NVLOG

NVLOG

NVLOG

NVLOG

NVLOG

NVRAM Full

WAFL

Cache



Module Summary

In this module, you should have learned to:

WAFL plans and manages storage

RAID protects data

The storage layer manages data transfer to

and from disk



Exercise

Module 13: Write and Read Request

Processing

Estimated Time: 10 minutes



Answers

Module 13: Write and Read Request

Processing



Check Your Understanding

What is a consistency point?

– A completely self-consistent image of the entire

file system

What is the purpose of RAID?

– To protect data—RAID striping is the methodused to protect disk data by which a parity value

is calculated across the disks in 4-Kb blocks

What is the storage layer?

– Handles the I/Os to disk and checks for disk

failures

M13 Writes-Reads FINAL

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