Questions regarding the details of differential backup

博雄 胡 765 Reputation points
2026-05-29T08:21:10.9233333+00:00

According to the online documentation, differential backup determines whether a sector has changed. I have the following two questions:

Is it possible that within a single sector, 1 page belongs to SQL Server and 7 pages belong to other application files (e.g., test.txt)?

If question 1 is true, and I do not modify that 1 page belonging to SQL Server but modify the content of test.txt, will it be considered as "changed" (i.e., having a difference) by the differential backup?

SQL Server Database Engine
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Answer accepted by question author
Erland Sommarskog 137.6K Reputation points MVP Volunteer Moderator
2026-05-29T12:11:56.2933333+00:00

First, are you asking about differential backups in SQL Server or in Windows? I ask, because sector is not a word used in SQL Server parlance. Seeing you posting alias, I realise that this can be a translation issue. Since you talk about 1+7 pages, I will assume the latter. The word you should have used is extent.

No, it is not possible. SQL Server operates within its own file in the file system, and the pages are defined with in that file. So, no, other files could be included an extent. And thus, your followup question does not apply.

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  1. AI answer

    2026-05-29T08:21:21.7933333+00:00

    SQL Server differential backups do not work at the physical disk-sector level and do not look at other files like test.txt.

    1. About “one sector with 1 SQL page + 7 pages of other files”
      SQL Server organizes its data in 8‑KB pages, grouped into extents (8 contiguous pages). Differential backups are based on SQL Server extents inside SQL Server data files, not on raw disk sectors or other files. The documentation states that a differential backup “captures the state of any extents … that have changed” and uses a bitmap page with a bit per extent to track changes. This bitmap and the extents it refers to exist inside SQL Server database files only. They do not span into unrelated files on disk. Therefore, the scenario “1 page belongs to SQL Server and 7 pages belong to other application files in the same sector” is not relevant to how SQL Server differential backups decide what to include.
    2. If only test.txt changes Because differential backups only consider SQL Server data extents (within .mdf, .ndf, etc.), changes to other files such as test.txt are ignored by SQL Server’s differential backup mechanism. The differential bitmap is updated only when SQL Server changes an extent in its own database files. If no SQL Server page in that extent is modified, the corresponding bit in the bitmap remains 0, and that extent is not included in the differential backup—regardless of what happens to other files on the same volume.

    So, modifying test.txt without modifying any SQL Server pages does not cause SQL Server to treat any of its extents as “different” for the purpose of a differential backup.

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