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SQL Server 2005 and Disk drive allocation unit size to 64K - any benefit or performance? - SQL Server Knowledge Sharing Network (@sqlmaster)

SQL Server 2005 and Disk drive allocation unit size to 64K - any benefit or performance?

You may be aware that or seen within your Database platform about slower disk performance than expect having multiple disks in Windows Server 2003 environment when you use a hardware-based redundant array of independent disks (RAID) or a software-based RAID. This could be due to one of the reasons such as starting location of partition is not aligned properly with a stripe unit boundary in the disk partition that is created on that RAID. Further information we can refer from one of the KBA929491 article that:

A volume cluster may be created over a stripe unit boundary instead of next to the stripe unit boundary. This is because Windows uses a factor of 512 bytes to create volume clusters. This behavior causes a misaligned partition. Two disk groups are accessed when a single volume cluster is updated on a misaligned partition.

Windows creates partitions that are based on a predefined number of sectors. The starting location for a disk partition in Windows Server 2003 is either the 32nd or the 64th sector, depending on the information that is presented to the operating system by the mass storage controller.

It is the default configuration from the disk partitions that they reserve the first sector for code and number of sectors with the starting sector information, thats how the next sector will consists the information about data part. In this regard Microsoft recommends to use DiskPart.Exe tool which can be used to create disk partition and specify the starting offset, before that it is recommended to findout what is the current size set of partitioned disks and you can get more information about the Diskpart.exe tool from this DiskPart-TechnetArticle and Exchange-Server_IO_Alignment article links.

Within the recent project tasks I have proposed to use disk partition as 64K blocks for the disks that are used for transaction logs, as there is no doubt it enhances the performance when writing the huge chunk of logs for a database (if required). Also I have discussed with Microsoft Development team about the recommendations they use internally with 64K for both data & log, further I suggest to refer Bob Dorr's explanation on how best system can make use of it during backup/restores link. One of my colleague (Ken) has raised the concern that "I can see that sometimes it would help but not always. My *guess* is that SQL won't always have 64k of data that needs to be written urgently (e.g. after a COMMIT). I guess it all depends on the algorithm's used and I've not seen them described in detail anywhere".

Further I have found that in the past when the Infrastructure team raised a query with Microsoft team it has been discourage to set 64K blocks to the drives other than Data disk. The usual disk layout seperation we follow for the SQL Server setup as follows: 

  • SQL binaries (System databases and software)
  • Tempdb (and Tempdb logs)
  • Application Database data files
  • Application Database transaction log files
  • Database Backups (system & user database)

Going forward I strongly suggest to use 64K blocks for Data, Log  and backup drives in order to take maximum performance, to support this I would like to quote the text from this Pre-Deployment I/O Best Practices article as:

NTFS Allocation Unit Size

When formatting the partition that will be used for SQL Server data files, it is recommended that you use a 64-KB allocation unit size for data, logs, and tempdb. Be aware however, that using allocation unit sizes greater than 4 KB results in the inability to use NTFS compression on the volume. SQL Server, although it is not recommended that you use this, does support read-only data on compressed volumes.

Published Monday, April 28, 2008 4:28 PM by ssqa.net


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Monday, April 28, 2008 8:42 PM by TrackBack