Dismiss
InnovationQ will be updated on Sunday, Oct. 22, from 10am ET - noon. You may experience brief service interruptions during that time.
Browse Prior Art Database

Load balancing of destage operations in a N way, write cache, disk subsystem

IP.com Disclosure Number: IPCOM000014097D
Original Publication Date: 2001-Mar-01
Included in the Prior Art Database: 2003-Jun-19
Document File: 4 page(s) / 86K

Publishing Venue

IBM

Abstract

A program is disclosed which improves the performance of a multi host or multi adapter disk subsystem that uses write caching. Destage operations are required in such a system, and this invention ensures that the destage operations are carried out by the most lightly loaded adapters in the system. This helps to maximise the overall performance of the system. Background Figure 1 shows a typical, 2 way disk system, in which 2 hosts are able to read and write data to a number of shared

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 36% of the total text.

Page 1 of 4

Load balancing of destage operations in a N way, write cache, disk subsystem

A program is disclosed which improves the performance of a multi host or multi adapter disk subsystem that uses write caching. Destage operations are required in such a system, and this invention ensures that the destage operations are carried out by the most lightly loaded adapters in the system. This helps to maximise the overall performance of the system.

Background

    Figure 1 shows a typical, 2 way disk system, in which 2
hosts are able to read and write data to a number of shared
disks. The disks are managed by an adapter in each host.

--------------- ---------------
| Host 1 | | Host 2 |
| | | |
|-------------| |-------------|
| Adapter 1 |----------- disks ------------| Adapter 2 |
|-------------| |-------------|
| | | |
--------------- --------------- Figure 1 -
A 2 way disk subsystem

    In a certain class of these two way disk systems, a write
cache is provided in both adapters, to improve the write response
time seen by the host. This means that the adapter can indicate
to the host that a write operation has completed as soon as the
customer data associated with that write has been copied into the
adapter write cache. The adapter has the responsibility of
destaging this data to the disks at a later date, without host
involvement or knowledge.

    In a dual active, 2 way system, read and write operations
may be concurrently performed from both hosts. This often has
performance and robustness advantages, since, compared toa1way
system, there are twice as many adapters to perform i/o and if
one host or adapter fails, the other host can immediately provide
an alternative i/o route to the data on the disks.

    The data in the write cache is shadowed between the two
adapters. If one adapter suffers a hardware failure before the
data in the write cache has been destaged to disk, no customer
data is lost, because that data can be written to disk from the
second adapter. Since the customer data is cached in two
adapters, careful synchronisation is needed between the two
adapters, to ensure proper operation. The details involved in a
write operation from host 1 are as follows:

Host 1 instructs adapter 1 to perform a write operation, at a certain LBA for a certain


1.


2.

length. Adapter 1 sends a message to adapter 2 informing adapter 2 that a write is to be

performed on disk 1, for that particular LBA and length. This message is part of a protocol that allows adapter 1 to obtain a 'lock' for this range of blocks on disk 1.

1

Page 2 of 4


3.


4.


5.


6.


7.

Adapter 2 will respond with a message indicating if it agrees that adapter 1 can take

this lock. The lock can be granted if adapter 2 is not processing an i/o request for any portion of the blocks in the write command. If the lock cannot be granted then adapter 1 must retry at a later time (the details of this retry are not relevant here). When adapter 1 has obtained a lock, the data from the host can be transferred into

the write cache in adapter...