Oracle Observations

April 18, 2010

The dawn of the 4k sector hard drive and its impact for the DBA.

Filed under: 11g,ASM — bigdaveroberts @ 12:47 pm

Firstly, I should state that, despite the fact that the launch of the new standard 4K (or ‘Advanced format’ drives as they are also known) were originally slated to co-inside with the release of Windows Vista (the first operating system to support them) in 2006, I have to admit that the first I heard of them was from an issue of a Custom PC article earlier this year, just before their belated, imminent launch.

Secondly, I would also suspect that as only one manufacturer (Western Digital) currently has product available and other manufacturers might have differing approaches to how they handle the issue of backward compatibility, chances are this post will at least in part be out of date fairly quickly!

Fundamentally, there will be two main areas in which the DBA may need to be aware.

1. Support for older operating systems.

2. ASM support.

Operating system support.

At the time of writing, to my knowledge, only 3 operating systems natively support the 4k standard, (Windows (Vista, 7 & Server 2008), MacOS (10.4 and later) and Linux (2.6.31 kernel and later.), that and the suggestion that by the end of this year the majority of hard disks are expected to be based on the new 4K standard has lead to speculation that if you intend to support older operating systems into the medium term future you need to start stockpiling 512 byte sector hard drives.

The truth is somewhat more complex than this.

While newer operating systems can operate with hard disks of both standards, there has been an attempt to produce some sort of backwards compatibility for older operating systems such as Windows 2000 and XP.

In simple terms, there should be no issues with using 4K dives as non-bootable data drives, although I’m confident that attempting to mix 512 and 4K drives in a raid array would be foolish and I strongly suspect that it would be impossible. The complexities and problems are likely to be encountered when attempting to boot older operating systems from the 4K hard drives.

The first solution(s) to booting older operating systems from the Western digital Green drives consists of a drive compatibility setting and a software kludge.

However while with minimal additional effort you will be able to boot XP from a 4K hard drive, the solution is unlikely to be compatible enough to facilitate the installation of multiple operating systems on the same drive or the use of some boot loaders.

NB. I am informed that if windows Vista is installed on a second partition of a disk where the primary partition is XP, then Vista will replace the XP boot loader with its own. In that case, I suspect that it would be potentially possible, with Vista or 7 on the primary partition, then the boot loader installed might be sophisticated enough to boot XP installed on a secondary partition, but at the moment that is pure speculation.

ASM support

I struggled to find any information about ASM support for 4K hard drives until I eventually found this document:

Extending ASM to Manage All Data

Essentially, 4K hard drives will be supported from Oracle 11.2, where both the RDBMS version and the ASM instance version are required to be at this level.

The type of hard drive in use will need to be specified during the disk group creation and you will not be able to mix disk types in a single group.

Example of the new syntax option taken from the Oracle document:

CREATE DISKGROUP data NORMAL REDUNDANCY
FAILGROUP ctrl_a DISK
‘/devices/diska1’, ‘/devices/diska2’
FAILGROUP ctrl_b DISK
‘/devices/diskb1’, ‘/devices/diskb2’
ATTRIBUTE ‘compatible.asm’ = ’11.2’, ‘compatible.rdbms’ = ’11.2’,
‘sector_size’=’4096’;

I see no indication that mixing 4K hard drives in 512 byte compatibility mode and true 512K drives will be supported by Oracle, and the fact that different hard disk manufacturers may implement 512 byte compatibility using different mechanisms, I strongly suspect that it never will be.

Ultimately, there are potential issues for ASM environments that can’t be upgraded to Oracle 11.2 in the medium term.

For those that can migrate, there are still potential issues where ASM disk groups using 512 byte disks will have to be migrated to disk groups based on 4K hard disks over time, potentially increasing complexity during recovery operations, and forcing administrators to manage 2 inventories of spare hard dives.

Final observations.

While the goals of 4K sector drives is to increase the areal density and maximum capacity, neither of these goals will be achieved with the first generation of new disks, as initially the areal density is unchanged (The extra capacity is currently left as extra space at the end of sectors.), there is no great value in being an early adopter of this technology apart from for research purposes.

While SCSI 4K disks will all be standardised, because SCSI has inbuilt mechanisms to allow the operating system to interrogate the underlying structure of the disk facilitating a standard approach, there is likely to be a variation in the approaches taken by hard disk manufacturers to SATA/IDE hard drives, which would discourage me from mixing disks from different manufacturers and would lead me to scour Metalink for patches before entrusting my data to a non-scsi 4K ASM disk array.

Ultimately, there could be potential issues.

Additional information.

An overview of the Western digital approach to backwards compatibility along with benchmarks:

bit-tech.net: The Facts: 4K Advanced Format Hard Disks

And article regarding formatting SSDs indicating that the type of issues encountered with 4K hard drives is not limited to this technology:

thunk.org: Aligning filesystems to an SSD’s erase block size

Discussion thread on the Linux kernel mailing list archive:

ATA support for 4k sector size

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