Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: FAT32 superiority over ext2 :-) | Date | Mon, 25 Jun 2001 02:03:07 +0200 |
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On Monday 25 June 2001 01:49, Albert D. Cahalan wrote: > Daniel Phillips writes: > > On Monday 25 June 2001 00:54, Albert D. Cahalan wrote: > >> By dumb luck (?), FAT32 is compatible with the phase-tree algorithm > >> as seen in Tux2. This means it offers full data integrity. > >> Yep, it whips your typical journalling filesystem. Look at what > >> we have in the superblock (boot sector): > >> > >> __u32 fat32_length; /* sectors/FAT */ > >> __u16 flags; /* bit 8: fat mirroring, low 4: active fat */ > >> __u8 version[2]; /* major, minor filesystem version */ > >> __u32 root_cluster; /* first cluster in root directory */ > >> __u16 info_sector; /* filesystem info sector */ > >> > >> All in one atomic write, one can... > >> > >> 1. change the active FAT > >> 2. change the root directory > >> 3. change the free space count > >> > >> That's enough to atomically move from one phase to the next. > >> You create new directories in the free space, and make FAT > >> changes to an inactive FAT copy. Then you write the superblock > >> to atomically transition to the next phase. > > > > Yes, FAT is what inspired me to go develop the algorithm. However, two > > words: 'lost clusters'. Now that may just be an implemenation detail ;-) > > What lost clusters? > > Set bit 8 of "flags" (A_BF_BPBExtFlags to Microsoft) to disable > FAT mirroring. Then the low 4 bits are a 0-based value that > indicates which copy of the FAT should be used. > > Assume we have 2 copies of the FAT, as is (was?) common. I'll call > them X and Y. When we mount the filesystem, we disable FAT mirroring > and mark FAT X active. > > Now we can make changes to FAT Y without affecting filesystem > integrity. Windows will not use FAT Y. As is usual with the > phase-tree algorithm, we use free space to create a new structure > beside the old one. > > Time for a phase change: > > We have FAT Y, currently inactive, updated on disk. > FAT X is active; it describes the current on-disk state. > We have a new root directory on disk, sitting in free space. > We have a new filesystem info sector on disk, sitting in free space. > > We write one single sector, then: > > FAT X becomes inactive, and will not be used by Windows. > FAT Y becomes active; it describes the new on-disk state. > The old root directory is marked free in FAT Y. Good! > The old filesystem info sector is marked free in FAT Y. Good! > > Once the superblock goes to disk, FAT X may be written to.
When can we expect the patch?
-- Daniel - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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