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Deciding on the right cluster size for an NTFS hard disk

16 May 2007

A cluster is a disk-storage unit consisting of a fixed number of sectors that the operating system uses to read or write information. Even for a 1-byte file, one cluster must still be allocated in a FAT file system. In NTFS, if a file is this small, it can be stored in the MFT record itself without using additional clusters. When a file grows beyond the cluster boundary, another cluster is allocated. It means that the bigger the cluster size, the more disk space is wasted, however, the performance is better.

These days, practically all new computers come with very large hard drives for storing information, and that information takes up more and more space, which means the average file size has grown too. Sooner or later, advanced users ask themselves what the optimal cluster size is for their hard disks. Windows recommends to use the defaults (see below), and in most cases this choice is the best.

The following table shows the default values that Windows NT/2000/XP/Vista uses for NTFS formatting:

Drive size
(logical volume)
Cluster size Sectors
512 MB or less 512 bytes 1
513 MB - 1,024 MB (1 GB) 1,024 bytes (1 KB) 2
1,025 MB - 2,048 MB (2 GB) 2,048 bytes (2 KB) 4
2,049 MB and larger 4,096 bytes (4 KB) 8

However, when you format a partition manually, you can specify cluster sizes of 512 bytes, 1 KB, 2 KB, 4 KB, 8 KB, 16 KB, 32 KB and 64 KB in the Format dialog box or as a parameter to the command-line FORMAT utility.

It is not recommended to use non-default values for system disks, as this can negatively affect system performance or cause software compatibility problems (some user reports on the internet, for instance, suggest that not all disk defragmentation programs support cluster sizes of 16 KB and higher).

The above statement would lead one to believe that non-default cluster sizes are only good for non-system disks. However, the opposite can be true in some cases and actually increase system performance. Besides, there are several factors that affect NTFS performance: cluster size, location and fragmentation of the Master File Table (MFT) and paging file, NTFS volume compression, NTFS volume origin (created or converted from existing FAT volume), not to mention drive type, RPM, system bus speed, etc.

We shall not stage any experiments as simple reasoning will show that, performance-wise, it is better to use bigger clusters (16 KB to 32 KB) for storing large multimedia files (e.g. music or video) or zipped archives (e.g. backups):

fewer clusters to read/write → fewer entries in the relevant table → less work for the hardware → greater speed.

Two notes, however: * If you plan to shrink older files to save disk space using NTFS compression, you had better leave cluster size at 4K. * The smaller your files (compared with the cluster size), the more fragmented your volume will tend to become over time.

Related file system performance tips

Defragment Drives Regularly

Fragmented drives increase the time it takes for applications to open, close, create or delete files. A good practice is to use Windows XP's Disk Defragmenter tool to defrag your drive at least once a week, especially if you run applications that frequently modify files and you have a lot of files on your drives. If you like you can use the Scheduled Task Wizard to automate this process. See How to Automate Disk Defragmenter Using Task Scheduler Tool in Windows XP (http://support.microsoft.com/default.aspx?scid=kb;en-us;555098) in the Microsoft Knowledge Base for instructions.

Converting from FAT to NTFS

If you did not setup Windows on a recently created NTFS Volume but converted the volume from FAT to NTFS, typically it causes MFT fragmentation so, generally, converted partitions are a lot slower than the ones originally created as NTFS. Defragmentation software is usually unable to defragment the MFT, however, you may be able to backup the whole system, reformat the volume with the right cluster size and put the system back again.

Compressing NTFS Volumes

Compression could save a certain amount of space on your volume and consequently increase or decrease your system's overall performance depending on the CPU speed, volume size, and data compressibility. If you have a fast CPU but a pretty slow HDD, compression is recommended because compressed data takes up less disk space and will be read and decompressed in the memory much faster than reading whole uncompressed blocks from the slow drive. The larger the volume (> 8GB), the poorer its performance if compressed. And, for sure, it is no use compressing a volume or folder containing uncompressible data such as JPG images, ZIP files, and the like. Ideal substance for compression is text and office documents, bitmap images, and other files consisting of lots of repeating strings of bytes.

To compress a file/folder/volume just go to its Advanced Attributes in Windows Explorer and mark the Compress… checkbox.

Paging File Defragmentation

You cannot use standard defragmentation tools to defragment the paging file (PAGEFILE.SYS) because it is being constantly used by the operating system for virtual memory storage. However, you can still do it manually. Just change its location to another drive (or decrease its size to a minimum if there is no other drive), reboot the machine, perform volume defragmentation, and change the parameters of the paging file back again. That's it!

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