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FAT and exFAT file systems

Windows Phone supports SDHC cards in a FAT or exFAT file system format. If the card is unformatted, or if the card contains an unsupported format such as NTFS, the system formats the SD card according to the SD specification if the user accepts the change at a format prompt. When this occurs, an SDHC card less than or equal to 32 GB in size is formatted in a FAT file system format.

This means that Windows Phone supports:

  • Up to 32-GB SDHC cards using the FAT file system.

  • Up to 2-terabyte SDXC cards using the exFAT file system.

When developing applications, consider the following FAT and exFAT file system limitations:

  • Linear directory search – Directory search for files in both FAT and exFAT file systems is linear, so don’t put too many files in the same directory. Users can sideload content, and they might choose to put all of their content in a single directory. The internal user store does not have this limitation because it is formatted in NTFS file system format.

  • File size limitation – File size is limited to 4 GB in FAT file system volumes. Your application still must support regular FAT file system volumes for sideloading scenarios.

  • Write-through caching – The SD card operates in write-through caching mode to minimize the risk of corruption and data loss. This means that writes to the file system are immediately written to storage. Applications should write their data in large, aligned chunks. The internal user store operates in write-back mode; writes are written to the cache and then written to storage later.

  • No ACLs – FAT and exFAT file systems do not support access control lists (ACLs), so you must assume that all of the content on the SD card is accessible by all applications.

Related topics

Expandable storage

 

 

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