Date: Sat, 25 Mar 2000 10:44:56 +0100 (MET) From: "Martin v.Loewis" <loewis@cs.tu-berlin.de> To: krentel@dreamscape.com Cc: freebsd-fs@FreeBSD.ORG, kwc@world.std.com Subject: Re: ext2fs optional features Message-ID: <200003250944.KAA29526@rubel.cs.tu-berlin.de> In-Reply-To: <200003250310.WAA00537@dreamscape.com> (krentel@dreamscape.com) References: <200003250310.WAA00537@dreamscape.com>
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What are the optional features? What does "sparse_super" do? Does Linux actually use these features, or are they for future use? Ext2 has three feature sets: compatible features, r/o compatible features, and incompatible features. If an ext2 implementation sees a volume that has a feature it does not recognize, it should act accordingly: If the feature is compatible, go ahead an mount the volume. If the feature is r/o compatible, refuse to mount r/w. If the feature is incompatible, refuse to mount at all. Currently (e2fstools 1.18), the following features are defined #define EXT2_FEATURE_COMPAT_DIR_PREALLOC 0x0001 #define EXT2_FEATURE_COMPAT_IMAGIC_INODES 0x0002 #define EXT3_FEATURE_COMPAT_HAS_JOURNAL 0x0004 #define EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER 0x0001 #define EXT2_FEATURE_RO_COMPAT_LARGE_FILE 0x0002 #define EXT2_FEATURE_RO_COMPAT_BTREE_DIR 0x0004 #define EXT2_FEATURE_INCOMPAT_COMPRESSION 0x0001 #define EXT2_FEATURE_INCOMPAT_FILETYPE 0x0002 #define EXT3_FEATURE_INCOMPAT_RECOVER 0x0004 The sparse_super option means that not every block group has a super block, but only those that are powers of 3, 5, or 7, and block group 0. The feature is ro-compatible, since an implementation can mount the file system when it finds a valid super block; it is not compatible, since the implementation will overwrite data when it attempts to write-back the super blocks into groups where none belong. Of the features above, Linux 2.3.99pre2 supports the following ones: #define EXT2_FEATURE_COMPAT_SUPP 0 #define EXT2_FEATURE_INCOMPAT_SUPP EXT2_FEATURE_INCOMPAT_FILETYPE #define EXT2_FEATURE_RO_COMPAT_SUPP (EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER| \ EXT2_FEATURE_RO_COMPAT_LARGE_FILE| \ EXT2_FEATURE_RO_COMPAT_BTREE_DIR) Whether these features are activated on a certain installation primarily depends on the default settings that the distributor (RedHat, Debian, ...) has selected. Regards, Martin To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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