From d79f499eae6671a04e80b5a3081fae8e2a311b54 Mon Sep 17 00:00:00 2001 From: Chris Mason Date: Mon, 8 Dec 2008 17:00:31 -0500 Subject: Btrfs: move data checksumming into a dedicated tree Btrfs stores checksums for each data block. Until now, they have been stored in the subvolume trees, indexed by the inode that is referencing the data block. This means that when we read the inode, we've probably read in at least some checksums as well. But, this has a few problems: * The checksums are indexed by logical offset in the file. When compression is on, this means we have to do the expensive checksumming on the uncompressed data. It would be faster if we could checksum the compressed data instead. * If we implement encryption, we'll be checksumming the plain text and storing that on disk. This is significantly less secure. * For either compression or encryption, we have to get the plain text back before we can verify the checksum as correct. This makes the raid layer balancing and extent moving much more expensive. * It makes the front end caching code more complex, as we have touch the subvolume and inodes as we cache extents. * There is potentitally one copy of the checksum in each subvolume referencing an extent. The solution used here is to store the extent checksums in a dedicated tree. This allows us to index the checksums by phyiscal extent start and length. It means: * The checksum is against the data stored on disk, after any compression or encryption is done. * The checksum is stored in a central location, and can be verified without following back references, or reading inodes. This makes compression significantly faster by reducing the amount of data that needs to be checksummed. It will also allow much faster raid management code in general. The checksums are indexed by a key with a fixed objectid (a magic value in ctree.h) and offset set to the starting byte of the extent. This allows us to copy the checksum items into the fsync log tree directly (or any other tree), without having to invent a second format for them. Signed-off-by: Chris Mason --- convert.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'convert.c') diff --git a/convert.c b/convert.c index 5a3c1413..388c6c44 100644 --- a/convert.c +++ b/convert.c @@ -2172,7 +2172,7 @@ int do_convert(const char *devname, int datacsum, int packing, int noxattr) { int i, fd, ret; u32 blocksize; - u64 blocks[6]; + u64 blocks[7]; u64 total_bytes; u64 super_bytenr; ext2_filsys ext2_fs; @@ -2195,7 +2195,7 @@ int do_convert(const char *devname, int datacsum, int packing, int noxattr) fprintf(stderr, "filetype feature is missing\n"); goto fail; } - for (i = 0; i < 6; i++) { + for (i = 0; i < 7; i++) { ret = ext2_alloc_block(ext2_fs, 0, blocks + i); if (ret) { fprintf(stderr, "not enough free space\n"); -- cgit v1.2.3